The correct answer is A. 5 to 10 cm.
A raft slab is a type of concrete foundation that is used to support heavy loads. It is a continuous slab that is poured on the ground and extends beyond the outer walls of the structure. This helps to distribute the weight of the structure evenly and prevents it from sinking into the ground.
The amount of projection beyond the outer walls of the structure depends on the type of soil and the weight of the structure. In general, the projection should be at least 5 to 10 cm. This will help to ensure that the raft slab is able to support the weight of the structure without sinking into the ground.
Option B is incorrect because it is too large. A projection of 15 to 20 cm would be excessive and would not be necessary in most cases.
Option C is incorrect because it is too small. A projection of 25 to 30 cm would not be enough to support the weight of the structure in most cases.
Option D is incorrect because it is too large. A projection of 30 to 45 cm would be excessive and would not be necessary in most cases.