Answer:
C is the correct answer
Explanation:
There is no "work." The question is a reading comprehension question, so the work goes on in your brain, where the words are interpreted. The problem statement tells you ...
"... the amount of calories he has used, f(d), is a function of the number of days, d, he has exercised with the new routine."
You are comparing this wording to the answer choice wording. The best match is to C (as you know), which says ...
"f(15) = 5250 means after 15 days of exercising with his new routine, Vincent has used 5250 calories."