C). After they cleaned their rooms, the boys planned to go swimming, but it started to rain.
Explanation:
A compound-complex sentence as the name suggests brings the compound and complex sentences together that contains at least three minimum clauses which make it a complicated sentence that is used to convey longer thoughts. It carries at least two coordinate independent clauses with one dependent clause at least, joined with a coordinate and subordinate conjunction or a relative pronoun. According to the questions, <u><em>option C</em></u><em> seems to perfectly justify the above claim as it contains two independent clauses, 'the boys planned to go swimming' and 'it started to rain' and one dependent clause 'After they cleaned their rooms' joined with coordinating conjunction 'but'.</em>
