You have to find the least common multiple of 12 and 15, which is 60 (12x5 = 60, 15x4 = 60). Therefore, the answer to 17 is D on the hour only. Since there are 60 mins in an hour, there are no other possible times during the hour that the trains could arrive at the same time.
To find a common denominator, you also need to find the least common multiple of the denominator of the fractions. The least common multiple of 4 and 7 is 28. 2/7 then becomes 8/28 and 3/4 becomes 21/28. The answer is A.