Answer:
Primary Reinforcement.
Explanation:
It is not Token Economy - that does not make sense in this context.
It is not Secondary Reinforcement - an example of this is giving a dog a treat and telling them "good dog!" at the same time. In this question, we are only giving a snack to the child.
It is not Reinforcement contingency because that describes the relationship between the reinforcer and the organism being reinforced.
It must be Primary Reinforcement. Food (if not paired with anything else) is an example of a primary reinforcer.