The correct answer is A. 25 kmph.
Single line working is a railway operating procedure used when a section of double track is temporarily unavailable for use. This can happen for a variety of reasons, such as track maintenance, repairs, or inclement weather. When single line working is in effect, trains can only travel in one direction on the track. This means that trains must pass each other at designated passing loops.
In fogy weather, visibility is reduced, which makes it difficult for drivers to see oncoming trains. To reduce the risk of collisions, the speed limit for trains is reduced when single line working is in effect during fogy weather. The speed limit is typically 25 kmph.
Option B is incorrect because it is the speed limit for trains in normal weather conditions. Option C is incorrect because it is the speed limit for trains in single line working conditions when there is no fog. Option D is incorrect because it is not a speed limit that is typically used in railway operations.