The correct answer is: A. Next whole block section.
A signal overlap is a situation where two trains are in the same block section at the same time. This can happen if a train is delayed and the signal ahead of it is not cleared in time. To prevent signal overlaps, advanced starters are used at A’ class stations. Advanced starters are signals that are located before the main signal. They allow trains to start moving before the main signal is cleared, which prevents signal overlaps.
If there is no advanced starter at an A’ class station, then the next whole block section will be used as the advanced starter. This means that the train will not be allowed to start moving until the next whole block section is clear. This will help to prevent signal overlaps and keep trains safe.
The other options are incorrect because they do not describe the correct way to prevent signal overlaps. Option B, back whole block section, is incorrect because it would allow the train to start moving before the previous block section is clear. This could lead to a signal overlap. Option C, block sectiond, is incorrect because it is not a specific term that is used in railway signaling. Option D, None of these, is incorrect because it is not the correct answer to the question.