Choose the correct form. I am not sure that will do his _____ at the exam since he is ill. a) letter b) test c) bestd)

I am not sure that he will do his best at the exam since he is ill.

Here we use the adjective best because it is a phrase as to do his best and it is translated in other way. That’s why we use that adjective. 

For example: She is doing battle with her friend. 

In this case to do battle is also a phrase.