The correct answer is: A. overtaking sight distance.
Overtaking sight distance is the distance required to safely overtake another vehicle. It is calculated by taking into account the speed of the overtaking vehicle, the speed of the overtaken vehicle, and the reaction time of the driver.
The desirable length of overtaking zone as per IRC recommendation is equal to overtaking sight distance. This is because it is important to have enough space to safely overtake another vehicle, without having to worry about oncoming traffic.
Option B is incorrect because it is twice the overtaking sight distance. This would not be enough space to safely overtake another vehicle, and would increase the risk of an accident.
Option C is incorrect because it is three times the overtaking sight distance. This would be more than enough space to safely overtake another vehicle, but it would also waste space on the road.
Option D is incorrect because it is five times the overtaking sight distance. This would be far too much space to safely overtake another vehicle, and would waste a lot of space on the road.