The correct answer is A. 25 mm.
Differential settlement is the difference in the vertical displacement of two points on a structure. It is a common problem in foundations on sandy soils, and can cause cracking and other damage to the structure.
The maximum differential settlement that can be tolerated without causing damage to a structure depends on a number of factors, including the type of structure, the soil type, and the depth of the foundation. In general, however, differential settlement should not exceed 25 mm.
Option B (40 mm) is too high, and could cause damage to the structure. Option C (65 mm) is also too high, and could cause significant damage to the structure. Option D (100 mm) is much too high, and would almost certainly cause the structure to collapse.
It is important to note that the maximum differential settlement that can be tolerated will vary depending on the specific circumstances. If you are unsure about the maximum differential settlement that can be tolerated for your particular structure, you should consult with a structural engineer.