Question

What information in an essay assignment can help you figure out the purpose of your writing?

A. Evidence
B. Form
C. Key words
D. Length requirements

Answer 1

Like I said before, key terms usually tell you what to research.
Length requirements isn't it.
I don't think form is it either.
So, what is your answer?

Answer 2

I think (evidence) because finding evidence may show what type of purpose is needed

Answer 3

It could equally be key terms or evidence to me.
However, you have to compile the evidence yourself, most times. Therefore, the key terms in the prompt or whatever you're given would normally tell me what my purpose of writing is.
But it is your choice so go with what you think.