Correct answer:
a. dijo
Since el entrenador can be replaced by the subject pronoun for the third person singular in masculine form, which is él (he), we must conjugate the verb decir matching this person as well. So this conjugation is dijo. On the other hand, me is the indirect object pronoun that matches the first person singular, so the action of the verb in this sentence is being performed for me. Finally:
Pero el entrenador me dijo que hiciste muy bien