The correct answer is A. 15 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 soil, and can cause cracking and other damage to the structure. The maximum permissible differential settlement is usually limited to 15 mm, to avoid these problems.
Option B, 25 mm, is too high and could cause damage to the structure. Option C, 35 mm, is also too high and could cause damage to the structure. Option D, 45 mm, is much too high and would almost certainly cause damage to the structure.
It is important to note that the maximum permissible differential settlement will vary depending on the type of structure, the soil conditions, and other factors. It is always best to consult with a structural engineer to determine the specific requirements for a particular project.