Which one word substitute best describes a person who insists very strictly on certain rules or details?

Introduction / Context:
This one word substitution question focuses on people who insist strongly on something, often on following rules or paying attention to details. Such words are useful in descriptive writing and comprehension passages. The options include several character related nouns, and you must choose the one that best matches the idea of being very strict or particular about a rule or standard.

Given Data / Assumptions:

• The phrase in the stem is a person who insists on something.
• Option A stickler refers to someone who insists unreasonably or very strictly on something.
• Option B boaster is someone who talks with too much pride about achievements.
• Option C disciplinarian is a person who believes in enforcing strict discipline in general.
• Option D instantaneous is an adjective meaning happening at once, not a noun for a person.

Concept / Approach:
A stickler is usually followed by the preposition for, as in stickler for punctuality or stickler for detail. It clearly expresses the idea that the person insists that certain rules be observed without compromise. While a disciplinarian is also strict, the word refers more broadly to a person who enforces discipline, especially in schools or organisations. The question wording a person who insists on something fits stickler more precisely, because it captures the idea of being particular about specific rules or standards, not just discipline overall. The other options do not match this nuance.

Step-by-Step Solution:
Step 1: Match the phrase a person who insists on something with a dictionary like definition.Step 2: Recall that a stickler is defined as a person who is very insistently exacting about a certain quality, rule, or standard.Step 3: Consider option B boaster, which is someone who brags. This focuses on speech about achievements, not insisting on rules.Step 4: Consider option C disciplinarian. This person enforces discipline strongly, but the phrase in the question is more general and does not mention discipline.Step 5: Option D instantaneous is not even a noun but an adjective, so it cannot describe a person. Therefore, stickler is the most accurate one word substitute.

Verification / Alternative check:
We can verify by using each option in example sentences. If we say He is a real stickler for accuracy, the sentence clearly means he insists that details be correct. If we say He is a real boaster, the emphasis shifts to bragging, which is different. Calling someone a strict disciplinarian suggests they enforce classroom rules but does not necessarily convey the idea of being particular about one specific rule such as time or neatness. This contrast shows that stickler most closely matches the idea in the stem.

Why Other Options Are Wrong:
Boaster describes prideful speech but not insistence on a rule. Disciplinarian is related but narrower and usually refers to a role or profession, whereas the stem phrase is more general and fits the set expression stickler for. Instantaneous cannot be correct because it is not a noun and does not describe a person. These explanations eliminate options B, C, and D, leaving stickler as the only suitable match.

Common Pitfalls:
It is easy to confuse stickler with disciplinarian because both suggest strictness. The difference is that stickler is often used for specific standards, like a stickler for truth, while disciplinarian is used for people who enforce disciplinary rules over others. Remember common collocations such as stickler for detail, which appear frequently in reading passages, and you will quickly recall the correct meaning in exams.

Final Answer:
The best one word substitute is Stickler, because it describes a person who insists very strictly on certain rules or details.