Although the breed started out as a roman cattle dog, rottweilers have also worked as guard dogs, draft dogs, and police dogs. in which type of sentence structure is this sentence written?
a)compound-complex
b)compound
c)simple
d)complex

The correct answer is D. complex.
A complex sentence contains one independent clause and at least one dependent clause. In the sentence above, the independent clause is Rottweilers have also worked as guard dogs, draft dogs, and police dogs, whereas the dependent clause (the one which cannot stand on its own) is Although the breed started out as a Roman cattle dog.