My reasoning is similar to selango. Let's start with the differences between B and C. Then let's look at assumptions.
P1: Almost every modern kitchen today is equipped with a microwave oven.
P2: Microwave ovens offer a fast and convenient way of cooking and reheating food.
P3: Microwave ovens have become a standard appliance in most households.
P4: Studies have shown, however, that microwave ovens are not completely safe and their use has occasionally resulted in serious injury.
C1: Some consumer advocates argue that microwave ovens should not be so readily accepted as a standard appliance until they can be certified to be completely safe.
Between B and C.
(B) "There have been many reported incidences of people who have been scalded by liquids superheated in microwave ovens." Incorrect. Does this answer strengthen the conclusion? It certainly doesn't hurt the argument, but it doesn't strengthen it either. The argument already mentions that "microwave ovens are not completely safe" (P4). This answer supports this premise but it doesn't add anything new to strengthen the conclusion.
The conclusion that "microwave ovens should not be accepted as a standard appliance until they can be certified to be completely safe" is still vulnerable to attack. It can be argued that even if microwave ovens are not completely safe they should still be a standard appliance in most households for other reasons such as convenience, speed, affordability, etc.
(C) Absolute safety is the only criterion by which an appliance should be judged to be acceptable as 'standard'. Correct. This answer redefines the term "standard". It places a much more strict definition on the term. Other factors such as convenience, speed, and affordability are no longer relevant. For an appliance to be considered "standard" it must be safe.
This answer strengthens the argument because it eliminates possible ways of attacking the conclusion. In the passage the conclusion is weak. It has the feel of an opinion. It seems like it is the consumer advocate's opinion that microwave ovens should not be a standard appliance because they are not safe. This answer has a way of transforming the conclusion form an opinion to a rule. Now, if an appliance does not meet this one criteria (absolute safety) then it cannot be considered a standard appliance.
ASSUMPTIONS
An assumption is an unstated premise. An assumption must be true for the conclusion to be true.
In this example, for the conclusion to hold, does it have to be true that "absolute safety is the only criterion by which an appliance should be judged to be acceptable as 'standard'"? No. Because the conclusion is not strongly worded it allows for some flexibility.
As mentioned above, the conclusion in the passage is more of an opinion. The consumer advocates can come to this conclusion even if (C) is untrue. In their opinion microwave ovens should not be a standard appliance because they are not safe. This is perfectly fine, however, their position is open to attack. One can argue that other factors such as convenience, speed, and affordability are more important than safety and therefore microwave ovens should be standard. The conclusion in the passage is valid but it is weak (this is probably why the question stem asks us to strengthen it).
Look at it another way. If (C) were an assumption then when we negate (C) the conclusion will not hold. Answer (C) negated becomes: "Absolute safety is NOT the only criterion by which an appliance should be judged to be acceptable as 'standard'". Does this destroy the conclusion. No. Even if there are criterion other than safety by which an appliance is judged, the conclusion still holds because it is not strongly worded. The consumer advocates still have a valid point that "microwave ovens should not be accepted as a standard appliance until they can be certified to be safe." They have a weak conclusion that's open to debate. Since the conclusion is not broken this is not an assumption.
Now consider this. What if the conclusion were more strongly worded? For example:
ACTUAL CONCLUSION: "Some consumer advocates argue that microwave ovens SHOULD NOT be so readily accepted as a standard appliance until they can be certified to be completely safe."
STRONGER CONCLUSION: "Some consumer advocates argue that microwave ovens CANNOT be readily accepted as a standard appliance until they can be certified to be completely safe."
In my opinion, if the stronger conclusion were substituted into the passage then answer (C) would in fact be an assumption of the argument. Because this conclusion is more strongly worded it is less flexible. For the conclusion to hold it must be true that "absolute safety is the only criterion by which an appliance should be judged to be acceptable as 'standard'".
I hope this helps clear up any confusion about assumptions...
