The correct answer is D. In the summer, school started at 6 a.m. and did not end until 5 p.m.
Step-by-step explanation:
In writing, combining two or more sentences imply including important information of the sentences while using appropriate conjunctions between them and avoiding unnecessary repetition. In the case of sentences " In the summer, school started at 6 a.m." and "School did not end until 5 p.m." it is not necessary to have two sentences as both are related to the schedule of the school instead as they can be linked using the conjunction "and" that shows they have the same importance and omitting the word "school" in the second sentence as it is not necessary. According to this, the best way to combine this sentences is " In the summer, school started at 6 a.m. and did not end until 5 p.m." as this links both sentences appropriately and preserves the meaning of both.