Assumption questions can be very tricky. The nice thing is that once you understand the format, the correct answer is usually more definite than on a normal Strengthen question.
We know that an assumption is an unstated premise used by the author to draw the conclusion. As a result, we can say that the assumption is a necessary condition; that is, the assumption must be true for the conclusion to be true. (As a simple example of a necessary condition, having 4 sides is a necessary condition for a shape to be a square. Obviously, it doesn't make the conclusion (being a square) true, but the conclusion can't be true without the necessary condition being met)
As we go through the answer choices, we can ask "what if this ISN'T true?" If we do that, the correct answer will destroy the argument. Here's a simple example:
People are wearing sunglasses, so it must be sunny out.
Which of the following is an assumption required by the argument?
A) Today's high temperature will be higher than 80 degrees Fahrenheit.
If this isn't true, then today's high temperature will be lower than 80 degrees Fahrenheit. Does this affect the conclusion? No.
B) The city is in the middle of a heat wave.
If this isn't true, then the city is NOT in the middle of a heat wave. Does this affect the conclusion? No.
C) People only wear sunglasses when it is sunny out.
If this isn't true, then people do not only wear sunglasses when it is sunny out. Does this affect the conclusion? Yes! The conclusion infers the relationship that sunglasses=sunny out; if we know this isn't true, then we cannot safely conclude that is sunny out.
