The complex sentence among the options is: "Cristina learned to swim when she was three, and now she is the captain of the high school swim team."
However, even though this sentence has two independent clauses ("Cristina learned to swim when she was three" and "now she is the captain of the high school swim team") connected by "and," it does not include a dependent clause.
The correct choice that fits the definition of a complex sentence is "Cristina learned to swim when she was three," because "when she was three" is a dependent clause.
So, the complex sentence is: "Cristina learned to swim when she was three."