Whenever you're asked to find the ASSUMPTION in CR, you must look for a logical disconnect between the premises and the conclusion.

Premises:
- Exposure to certain chemicals commonly used in elementary schools as cleaners or pesticides causes allergic reactions in some children.
- Elementary school nurses in Renston report that the proportion of schoolchildren sent to them for treatment of allergic reactions to those chemicals has increased significantly over the past ten years.

Conclusion:
Therefore, either Renston's schoolchildren have been exposed to greater quantities of the chemicals, or they are more sensitive to them than schoolchildren were ten years ago.

Logical disconnect:
Just because a greater proportion of children are sent to the nurse, does it necessarily follow that the children are more exposed/sensitive to these chemicals? Does the proportion of children sent to the nurse always perfectly reflect the proportion of children who are sick? (Or have behaviors / social norms about going to the nurse changed?) Do the nurses always report things accurately?

Assumptions:
For the conclusion to follow from the premises, the author must be assuming that:
a) Children who have allergic reactions are sent to the nurse at the same rate now as they were 10 years ago.
b) Nurses are reporting these instances just as accurately as they were 10 years ago.

Now let's look at the answer choices:

(A) The number of school nurses employed by Renston's elementary schools has not decreased over the past ten years.
Irrelevant. The number of nurses has no bearing on whether the students have more exposure to allergens.

(B) Children who are allergic to the chemicals are no more likely than other children to have allergies to other substances.
This conclusion only specifies exposure/sensitivity to THESE allergens. Other allergies are irrelevant to the argument.

(C) Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago.
Correct. This is the assumption that must hold true for the argument [proportion of kids sent to the nurse increasing = allergy exposure/sensitivity increasing] to hold.

(D) The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston.
The argument doesn't specify WHERE the exposure happened, so this doesn't have to be true for the argument to hold.

(E) Children attending elementary school do not make up a larger proportion of Renston's population now than they did ten years ago.
We don't care about the comparison of children to the rest of the population. That has no bearing on whether the proportion of children sent to the nurse reflects the proportion of children with a given allergy.

The answer is C.