The correct answer is: 40 kmph.
The maximum speed of trains during thick & foggy weather in absolute block system is 40 kmph when signal is showing green. This is because fog reduces visibility and makes it difficult for the driver to see the track ahead. At a speed of 40 kmph, the driver has enough time to react if they see an obstacle on the track.
Option A is incorrect because it is the maximum speed of trains during normal weather conditions. Option B is incorrect because it is the minimum speed of trains during normal weather conditions. Option C is incorrect because it is the maximum speed of trains during thick & foggy weather in the manual block system. Option D is incorrect because it is the maximum speed of trains during thick & foggy weather in the electric multiple unit (EMU) system.