I am pretty sure it is option "A."
Diligence by definition means to "careful and persistent work or effort"
So yeah the answer is A.
Hope this helped-- <span />
“i don’t know the answer.” is a clause
Dear (friend),
You should do your homework so that you understand the subject taught in class. If you don't do your homework, your grades will drop and you will not receive a good grade in the class.
Your friend,
(Name)
