The correct answer is C. 30 kmph.
The maximum speed of the assistance train to be sent on T/A 602 is 30 kmph. This is because the assistance train is required to travel at a speed that is not too fast, so as not to cause further damage to the train that is in trouble. However, the assistance train also needs to travel at a speed that is fast enough to reach the train in trouble in a timely manner. 10 kmph is too slow, and 40 kmph is too fast. Therefore, the correct answer is 30 kmph.
Option A: 10 kmph is too slow. The assistance train needs to travel at a speed that is fast enough to reach the train in trouble in a timely manner. 10 kmph is not fast enough, and it would take too long for the assistance train to reach the train in trouble.
Option B: 15 kmph is a bit too slow. The assistance train needs to travel at a speed that is fast enough to reach the train in trouble in a timely manner. 15 kmph is a bit too slow, and it would take a bit too long for the assistance train to reach the train in trouble.
Option C: 30 kmph is the correct answer. The assistance train needs to travel at a speed that is fast enough to reach the train in trouble in a timely manner, but not too fast so as not to cause further damage to the train that is in trouble. 30 kmph is a speed that is both fast enough and safe.
Option D: 40 kmph is too fast. The assistance train needs to travel at a speed that is fast enough to reach the train in trouble in a timely manner, but not too fast so as not to cause further damage to the train that is in trouble. 40 kmph is too fast, and it could cause further damage to the train that is in trouble.