Answer: C. Population size
The students are trying to simulate the effects on a food resource which different population sizes may have. Each number of students per week represent different population sizes.
Answer: C. Population size
Explanation:
Because it is mentioned that all factors were kept the same, except for "number of students allowed to collect cereal at the same time" and this represents a change in the number of members in a population, which determines the size of the popuation itself.