Which sentence is a complex sentence?

a. that book is heavy, or i would bring it with me.
b. after i left, maria called my cell.
c. she asked me for that book.
d. she was upset, but i refused.

Explanation:

A]

A is a compound sentence. Or joins 2 principle clauses.

That book is heavy. I would bring the book with me.

B]

B is your answer.

Principle Clause: Maria called my cell.

Subordinate Clause: After I left.

C]

She asked for that book is a principle clause. There's nothing else there. It is not a complex sentence.

D]

This is a compound sentence.

She was upset.

I refused.