as he said "The sentence that is punctuated correctly is D. Our grandparents are returning home by train; their departure time is 8:00 A.M. You need to use the semicolon here because these two sentences aren't that connected. You have to use either a semicolon or a conjunction, but definitely not both, which is why B is incorrect. A is incorrect because a comma is not enough here - either a semicolon or a conjunction would suffice. And obviously, C is incorrect because there is nothing connecting these two sentences".