The correct answer is C. complex.
A complex sentence contains one independent clause and at least one dependent clause. Here, the independent clause is Galileo, the first astronomer to use a telescope, confirmed the theory and the dependent clause is that the earth and planets circle the sun.
A is incorrect because a simple sentence contains only one independent clause; B is incorrect because a compound sentence contains at least two independent clauses; D is incorrect because a compound-complex sentence contains one independent clause and at least two dependent ones.
