The correct answer is: 15.
A block section is a section of railway track that is isolated from the rest of the railway by signals. This allows trains to pass each other safely. The maximum number of track machines that can be sent for working in a block section is 15. This is because each track machine needs a certain amount of space to operate safely. If there are too many track machines in a block section, they could collide with each other or with trains.
Option A: 20 is incorrect because it is more than the maximum number of track machines that can be sent for working in a block section.
Option B: 15 is the correct answer because it is the maximum number of track machines that can be sent for working in a block section.
Option C: 25 is incorrect because it is more than the maximum number of track machines that can be sent for working in a block section.
Option D: 30 is incorrect because it is more than the maximum number of track machines that can be sent for working in a block section.