Final answer:
The complex sentence among the options is 'Andy didn't join his friends at the pool because he couldn't swim,' as it includes both an independent clause and a dependent clause.
Step-by-step explanation:
The question asks which of the provided sentences is a complex sentence. A complex sentence consists of one independent clause and at least one dependent clause. Looking at the options given, the sentence 'Andy didn't join his friends at the pool because he couldn't swim' is the complex sentence because it has an independent clause 'Andy didn't join his friends at the pool' and a dependent clause 'because he couldn't swim', which is introduced by the subordinating conjunction 'because'.