Improve the bracketed part of the sentence by choosing the correct idiomatic preposition. Any bad habit must be nipped (on) the bud.

Introduction / Context:
This question tests knowledge of a fixed English idiom: to nip something in the bud. The sentence is about stopping a bad habit early before it develops fully. The task is to select the correct preposition to replace the bracketed word on so that the idiom is used properly. Mastering such set phrases is important because idioms usually do not allow much variation in their wording.

Given Data / Assumptions:
• The sentence given is: Any bad habit must be nipped on the bud.• The highlighted phrase is nipped on the bud.• We know that the standard idiom is nip something in the bud.• The meaning is to stop a problem at an early stage.

Concept / Approach:
The idiom comes from gardening. A bud is the early stage of a flower or leaf. If you nip or pinch off the bud, the plant will not develop that shoot or flower. Figuratively, to nip a problem in the bud means to prevent it from developing by dealing with it immediately when it is still small. The preposition in is essential in this idiom. Using on or off changes the structure and makes it sound wrong to a native speaker. Therefore, the correct improvement is to replace on with in.

Step-by-Step Solution:
Step 1: Recognise that the sentence is intended to use a well known idiom about bad habits and early intervention.Step 2: Recall the complete idiom nip in the bud, which always uses in.Step 3: Replace on with in to form the correct phrase: Any bad habit must be nipped in the bud.Step 4: Check that the resulting sentence is grammatically correct and meaningful.Step 5: Choose option b, which provides the correct preposition.

Verification / Alternative check:
Test each alternative in the sentence. Nipped on the bud does not correspond to any recognised idiom in English and sounds unnatural. Nipped off the bud describes a literal action in gardening but is not the standard figurative expression used in speech and writing about problems and habits. No improvement would preserve the incorrect phrase and therefore cannot be chosen. Only nipped in the bud matches the long established idiom that appears in dictionaries and formal writing.

Why Other Options Are Wrong:
Option a: on is incorrect because nip on the bud is not an idiomatic or common phrase.Option c: off describes removal in a physical sense, but the idiomatic expression used with bad habits and problems is nip in the bud, not nip off the bud.Option d: no improvement is wrong because the original sentence clearly misuses the preposition.

Common Pitfalls:
A common error with idioms is to remember the general image but not the exact wording, leading to near misses like on the bud or at the bud. Exam questions often target these almost correct versions. To avoid such mistakes, it is helpful to revise idioms as fixed chunks and notice the specific prepositions they take. Associating the image of a bud and the idea of something being inside that early stage can also help you remember that in is the correct preposition here.

Final Answer:
The correct idiomatic form is nip a bad habit in the bud, meaning to stop it at an early stage. Correct answer: in.