A sentence is given with an underlined part. Choose the option that best improves it, or select "No improvement" if the sentence is already correct: "Owing to his respiratory problems the doctor has told him to refrain from smoking."

Introduction / Context:
Sentence improvement questions check whether you can recognise correct standard English usage. In this item, the sentence is: Owing to his respiratory problems the doctor has told him to refrain from smoking. The phrase to refrain from is very common in formal and semi formal English, especially when giving advice or instructions. You must decide whether this structure is correct or if one of the alternatives offers a better, more grammatical version.

Given Data / Assumptions:

• The core expression is has told him to refrain from smoking.
• Refrain is followed by the preposition from and then a gerund (ing form).
• The alternatives suggest different preposition or particle placements.
• If the original is already standard and grammatical, No improvement must be selected.

Concept / Approach:
The verb refrain in standard English is almost always followed by the preposition from and then a noun or gerund phrase: refrain from talking, refrain from junk food, refrain from smoking. Using not with refrain usually changes position (for example, do not refrain from helping others). In advice or instructions, the pattern tell someone to refrain from doing something is perfectly correct and formal. Therefore, any alternative that alters from or disrupts this pattern is ungrammatical and should be rejected.

Step-by-Step Solution:
Step 1: Examine the original: has told him to refrain from smoking. This matches the standard pattern to refrain from + gerund. Step 2: Consider Option A: to not refrain from. This reverses the meaning, suggesting he should continue smoking, which is the opposite of the intended advice and also creates an awkward negative structure. Step 3: Consider Option B: to refrain to. This is grammatically incorrect because refrain is not followed by to plus bare infinitive in this sense. Step 4: Consider Option C: to refrain not from. This is a clumsy and unnatural word order in modern English and would confuse the meaning. Step 5: Recognise that the original sentence is already correct, and therefore choose No improvement.

Verification / Alternative check:
Consulting standard usage, you will find many examples such as The doctor advised him to refrain from alcohol, Please refrain from using mobile phones during the meeting, and Drivers must refrain from overtaking on bends. All of these follow the same pattern refrain from + noun or gerund. In contrast, constructions like refrain to smoke or refrain not from smoke do not occur in natural, educated English. This confirms that the given sentence uses the correct structure, and no change is required.

Why Other Options Are Wrong:
to not refrain from: This is wrong both grammatically and logically. It introduces a double negative like do not refrain, which would mean continue smoking, the opposite of the intended medical advice.
to refrain to: This is wrong because refrain is not followed by to in this meaning. The correct preposition is from, and the verb after it should be in the ing form.
to refrain not from: This is wrong because it creates a confusing double negative and does not reflect natural word order in modern English usage.

Common Pitfalls:
A common trap in such questions is to assume that any option that looks more complex or formal must be better. In reality, the simplest and most standard form is usually correct. Learners sometimes think they must change something because the question appears under sentence improvement, but exams often include examples where no correction is needed. Always check whether the original sentence already matches standard patterns before deciding to modify it.

Final Answer:
The original sentence is correct, so the best choice is No improvement.