Modal Verbs show us the attitude of the speaker to what is being said or done. The term "modal" means expressing mood and mood is a way to express the attitude of the speaker.

Option A is correct because it expresses a keen ability to act. The sentence is basically of the past, which means that has happened in the past. Therefore the verb applied is also of past form.

Whereas, option B expresses an ability in the present; option C expresses what needs to be done in the present and option D suggests what might be done in the future. None of these answers suits the given sentence. And thus, option B,C and D are incorrect.