The correct choice is (a) 25 kmph
The best I can explain: Brake test is done in cold condition as well as in hot condition. A brake is considered cold when its temperature is below 80°C. An artificial horizontal track is prepared, which gives good grip for tyres. It is desirable to test the brake at maximum speed or at 25 kmph, whichever is lower. Test should be done in ballasted and unballasted conditions.