The correct answer is: D. rule that can be broken or modified.
A hard and fast rule is a rule that is very strict and must be followed without exception. It is a rule that is not open to interpretation or modification.
Option A is incorrect because a hard and fast rule is not necessarily difficult. It may be easy to follow, but it is still a rule that must be followed without exception.
Option B is incorrect because a hard and fast rule is not necessarily fat changing. It may not have any effect on a person’s weight, but it is still a rule that must be followed without exception.
Option C is incorrect because a hard and fast rule is not necessarily a rule that cannot be broken or modified. It may be a rule that is difficult to break or modify, but it is still possible to do so.
In conclusion, the correct answer is: D. rule that can be broken or modified.