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.

The answer would be B.

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.