English Phrase — Select the correct definition. Sentence: There is no hard and fast rule regarding this subject.

Introduction / Context:
The fixed expression “hard and fast rule” means a rule that is strict, fixed, and invariable. The sentence states that such a rule does not exist for the subject at hand. To answer correctly, you must know the definition of the phrase itself, not the negation used in the sentence. Exams often frame such items to test whether you know the core meaning independently of the context’s polarity (“no hard and fast rule”).

Given Data / Assumptions:

• Focus expression: hard and fast rule.
• The sentence uses negation: “There is no …,” but we are asked for the meaning of the phrase.
• Options include both fixed and flexible interpretations.

Concept / Approach:
“Hard and fast” historically meant firmly fastened. In modern usage, it denotes something rigid and unchangeable. Therefore, the correct definition of a “hard and fast rule” is “a rule that cannot be broken or modified.” Even though the sentence says there is no such rule here, the dictionary meaning remains that the phrase describes strict invariability.

Step-by-Step Solution:

Verification / Alternative check:
Paraphrase the full sentence: “There is no rule here that is strict and unchangeable.” This is a common advisory line in manuals and classroom instructions when flexibility is allowed.

Why Other Options Are Wrong:

• rule that is difficult: Confuses rigidity with difficulty.
• rule that is fat changing: Contains a typographical error and contradicts “hard and fast.”
• rule that can be broken or modified: Opposite of “hard and fast.”

Common Pitfalls:
Letting sentence negation flip your answer choice. Even if “no” precedes the phrase, the question typically asks for the phrase’s meaning by itself; then you can interpret the full sentence afterward.

Final Answer:
rule that cannot be broken or modified