The correct answer is B. 25 mm.
Expansion joints are used in rigid pavements to allow for movement of the pavement due to temperature changes and other factors. The thickness of the expansion joint is important to ensure that it is able to accommodate the expected movement without cracking. A thickness of 25 mm is typically sufficient for most applications.
Option A is incorrect because it is too small. A joint that is too thin will not be able to accommodate the expected movement and will likely crack.
Option C is incorrect because it is too large. A joint that is too thick will be more expensive to install and will not provide any additional benefit.
Option D is incorrect because it is not a realistic option. A joint that is 100 mm thick would be very difficult to install and would not be practical in most applications.