For this I would choose the first choice, A. "Eileen bought pants but not a new jacket."
A complex sentence is formed by an independent and a dependent clause. An independent clause is a part of a sentence that can still be a sentence on it's own. For example, "Eileen bought pants." could be a valid sentence.
A dependent clause is the opposite, and is a part of a sentence that can not still be a sentence on it's own. "But not a new jacket." is not a valid sentence.
