Let the speeds of the faster trawler (T1) and the slower trawler (T2) be x and y kph respectively. Then the distance covered by T1 and T2 in 45 minutes (30+15) will be 3x/4 kms and 3y/4. Then, 3x/4 = 3y/4 + 21/2....> 3x = 3y + 42....> x = y + 14. Luckily, there is only one option (B) where the difference in speeds is 14 but because of option (E) "None of these", we can't straightaway opt for (B). So we must test this by ascertaining whether, basing on this values, T1 does indeed cover 10.5 kms more than T2 in 45 minutes. Distance covered by T1 = 15 + 7.5 = 22.5. Distance covered by T2 = 8 + 4 = 12. So, T1 does indeed cover 10.5 kms more than T2 in the 45 minutes after starting.
ANS: B
P.S. If there were more than one option with 14 km speed difference then each answer would have to be tested. Alternatively, you could substitute the value of x in terms of y (or vice versa) from (x=y+14) in the equation (x/2)^2 + (y/2)^2 = (17)^2 (Pythagoras) and get the value of x or y. But this is a cumbersome and time-consuming process mainly because of the large numbers involved.