The correct answer is B. 5.
A block section is a section of track that is isolated from the rest of the railway by signals. This allows trains to operate safely and efficiently. The number of track machines that are allowed to work within a block section is limited to 5. This is to ensure that there is no danger of collisions between trains and track machines.
Option A is incorrect because it allows for only 2 track machines to work within a block section. This is not enough to ensure that all track maintenance work can be carried out safely and efficiently.
Option C is incorrect because it allows for 8 track machines to work within a block section. This is too many track machines and could lead to collisions between trains and track machines.
Option D is incorrect because it allows for 10 track machines to work within a block section. This is also too many track machines and could lead to collisions between trains and track machines.