Question

The following is what type of sentence? dogs and cats sometimes fight in a veterinarian's wating room.

a. simple,
b. compound,
c. complex,
d. compound -complex

Answer 1

The given sentence above is an example of a simple sentence. It is a simple sentence because it only consists of one independent clause with a subject/s and verb/s. It contains a compound subject which is "dogs and cats" and a single predicate which is "fight". 

Answer 2

The sentence Dogs and cats sometimes fight in a veterinarian's waiting room is A) a simple sentence.
A simple sentence has only one independent clause, and this sentence has only one independent clause (only one verb) so it has to be simple.