Diet by training for the gamse?

Please look at this question.

"All members of the Baja basketball team are required to commit to a high-protein and carbohydrate diet ____ training for the national games"
(a) when (b) that (c) by (d) at

The correct answer is (a)when. I think )c) by can be another answer. It is right in both of grammar and context. Am I wrong?
Could you explain to me why (c)by can't be another answer to this question?

Thank you for your answers in advance.