The correct answer is: D. rule that can be broken or modified.
A hard and fast rule is a
rule that is absolute and cannot be broken or modified. In the context of the question, there is no rule that is absolute and cannot be broken or modified regarding the subject. This means that there is no rule that is set in stone and cannot be changed. There may be rules that are generally followed, but these rules can always be broken or modified if necessary.Option A is incorrect because a difficult rule is not necessarily a rule that cannot be broken or modified. A rule can be difficult to follow, but it can still be broken or modified if necessary.
Option B is incorrect because a fast changing rule is not necessarily a rule that cannot be broken or modified. A rule can change quickly, but it can still be broken or modified if necessary.
Option C is incorrect because a rule that cannot be broken or modified is a hard and fast rule. In the context of the question, there is no rule that is absolute and cannot be broken or modified regarding the subject.