First off, a compound sentence is a sentence that contains at least two independent clauses. That tells us C is out almost immediately because it has a dependent clause, "After Maria finished dinner". Next, is B, That's a simple sentence, there's no joining method or comma and it only has one subject and one predicate. D isn't right either, it just about the same as B. Your answer would be A. Paolo missed his girlfriend, but he knew she'd be back soon. This sentence has a joining clause, and two independent subjects.
Hope this helps,
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