But that does not explain the increased soda consumption !

If the students can now buy sodas for less money, it would explain why their soda consumption would increase

B is the answer

A) In the month before the new survey, several billboards in the school’s town displayed ads for popular brands of soda.-> Marketing a product doesn't guarantee an increase in consumption
B ) The school’s cafeteria, after removing the sodas with corn syrup, began offering sodas made from cane sugar at a lower price than the sodas made with corn syrup.-> Because there is syrup even at a lower price, this can lead to more consumption. At least for the initial month as the students would like to check with a cane sugar cane one.
C)Many families in the school district have at least one parent who works at the town’s biggest employer, a soda distribution plant.-> Working in a soda distribution plan doesn't guarantee that soda will be purchased from the school cafeteria .
D) In addition to buying sodas at the school cafeteria, students may also purchase sodas from one of the several vending machines on school property.-> the volume would have remained stagnant or reduced instead of increasing in consumption. As a matter of fact, the student switch to these vending machines.
E) To comply with state standards, the school recently implemented a new health program with a focus on nutrition.-> This weakens the premise.