The correct option is A Many Indian temples stand testimony to the high level of art that existed in bygone eras.
Many Indian temples is the subject of the sentence, it is being talked about, further proven by the following verb stand indicating the former doing the work in the given sentence. The rest of the words form the predicate, i.e., talk about the subject. Option A consists of the perfect arrangement of the subject and the predicate in terms of order and syntax thus making complete sense of the given sets of words. Hence, Option A is the correct answer.