Question

Which of the following is a complex sentence?
A. She was invited to the party, but she had already made other plans.
B. He wasn't hungry because he had a late lunch.
C. The dog followed her around the apartment all day long.
D. Initially, the conference seemed unimportant, but he was glad he attended

Answer 1

I think its B.) He wasnt hungry because he had a late lunch.

Answer 2

B is the correct answer

It has a an independent clause + a dependent clause.....

A and D  are compound sentences
C is a simple sentence

hope it helps !