Which of these options is a complex sentence?

A) As she was unable to swim, Lena didn't want to go to the pool party.

B) As she was unable to swim, Lena didn't want to go to the pool party, but Jasmin convinced her to go.

A) is a complex sentence

Complex Sentence consists of 1 independent clause and 1 dependent clause.

As she was unable to swim - dependent clause (it can't be a sentence on its own)

Lena didn't want to go to the pool party - independent clause (it can be a sentence on its own)

If a dependent clause comes before the independent clause, there has to be a comma joining the 2 clauses.