Why is the answer is option a. It is clearly mention that, there is no disruption.
If the trains are to meet, they must be synchronized: e.g., starting from 12:00 both meet, then at 12:30 (so twice an hour). If they are not synchronized, they will never meet: Victoria: 12:00, 12:30
Charing: 12:02, 12:17, 12:47 and so on.