The correct answer is A. 25 mm.
The maximum total settlement for isolated foundations on clayey soils should be limited to 25 mm. This is because clayey soils are relatively soft and can deform easily under load. If the foundation settles too much, it can cause the structure to crack or even collapse.
Option B, 40 mm, is too high. This would result in excessive settlement and could damage the structure.
Option C, 65 mm, is also too high. It would result in significant settlement and could cause problems with the structure.
Option D, 100 mm, is the highest of the options and would result in unacceptable settlement. It is not advisable to allow a foundation to settle this much.
In conclusion, the maximum total settlement for isolated foundations on clayey soils should be limited to 25 mm. This will help to ensure that the structure is not damaged and that it remains safe to use.