The correct answer is D. 4 km/h.
The speed of the following tower wagon in the block section is 4 km/h. This is because the block section is a short distance, and the tower wagon is moving at a slow speed. The other options are incorrect because they are either too high or too low.
Option A is incorrect because it is too high. The speed of the following tower wagon in the block section is not 5 km/h. This is because the block section is a short distance, and the tower wagon is moving at a slow speed.
Option B is incorrect because it is too low. The speed of the following tower wagon in the block section is not 2 km/h. This is because the block section is a short distance, and the tower wagon is moving at a slow speed.
Option C is incorrect because it is too high. The speed of the following tower wagon in the block section is not 8 km/h. This is because the block section is a short distance, and the tower wagon is moving at a slow speed.