The correct answer is C. 3 meters.
The danger zone in 25 KV AC traction is 3 meters from OHE. This is because the electric field strength at this distance is high enough to cause a shock hazard. If you are within this distance of an OHE, you could be electrocuted if you touch it or anything that is in contact with it.
Option A is incorrect because 1 meter is not far enough away from an OHE to be safe. Option B is incorrect because 2 meters is not far enough away from an OHE to be safe. Option D is incorrect because 4 meters is not far enough away from an OHE to be safe.
It is important to stay away from OHEs, even if they are not energized. This is because they can become energized at any time. If you are ever in doubt about whether or not it is safe to approach an OHE, it is best to err on the side of caution and stay away.