The correct answer, and the sentence that shows a complex sentence with a correctly indicated subordinate clause is C: I'm completely useless to everyone in the morning, until I've had my cup of coffee.
This sentence contains an independent clause that could stand as a sentence expressing a complete thought ("I'm completely useless to everyone in the morning") but also a subordinate clause that needs more information to express a complete idea ("until I've had my cup of coffee").
This subordinate clause is introduced by the conjunction "until", which reduces the importance of the information of the subordinate clause and implies a cause and effect relation with the main clause.
The other options available do not indicate the subordinate clause correctly, since "We plan to go to the swimming pool", "baby Drake is often crying", and "She's a wonderful dancer" are actually the main clauses of their respective sentences.
