The correct answer is A. 150 m.
The permissible rate of change of grade is 0.003 per 30 meters. This means that the grade can change by 0.003% for every 30 meters of track. In this case, the grade is changing from +0.08% to -0.07%. This is a change of 0.15%. The length of the transition curve is therefore 0.15% / 0.003%/30 m = 150 m.
Option B is incorrect because it is the length of the transition curve if the grade were changing from +0.08% to -0.07% at a rate of 0.002% per 30 meters.
Option C is incorrect because it is the length of the transition curve if the grade were changing from +0.08% to -0.07% at a rate of 0.004% per 30 meters.
Option D is incorrect because it is the length of the transition curve if the grade were changing from +0.08% to -0.07% at a rate of 0.005% per 30 meters.