This sentence is C. compound.
Compound sentences consist of at least two independent clauses, and each clause has to have at least one predicate, or verb. Here, there are two independent clauses: My uncle and I built (the treehouse in our back yard) AND (My uncle and I) painted the tree house in our back yard.
A simple sentence has only one verb; a complex sentences has to have a subordinate clause as well as independent one.
