The appropriate option is: A. The apostrophes with children, Smith, Powell and caretaker indicate belonging of books, room, office, and cupboard respectively. A colon may be used between independent clauses when the second sentence explains, illustrates, paraphrases, or expands on the first sentence. The colon after 'places' expands on the previous statement. Therefore, the correct option is: A) The children's books were all left in the following places: Mrs. Smith's room, Mr. Powell's office, and the caretaker's cupboard.