In the following sentence improvement question, choose the best version of the sentence: 'No sooner had he completed his first novel than he fell seriously ill.'

Introduction / Context:
This question tests knowledge of correlative conjunctions and inversion in English, specifically the structure with no sooner. Competitive exams often include this pattern because it combines an inverted auxiliary verb with the correlative pair no sooner and than. The sentence describes two events in close succession: completion of the first novel and falling seriously ill. The candidate must identify the option that follows standard, formal English usage for this expression.

Given Data / Assumptions:

• Original idea: Immediately after he completed his first novel, he fell seriously ill.
• The expression no sooner is used to emphasise how quickly one action follows another.
• Typical pattern in formal English: No sooner had + subject + past participle + than + clause.
• Options alter word order and the conjunction that follows no sooner.

Concept / Approach:
When no sooner is placed at the beginning of a sentence for emphasis, English requires inversion, meaning the auxiliary verb comes before the subject, similar to questions. The correct pattern is: No sooner had he completed his first novel than he fell seriously ill. The conjunction that pairs with no sooner is than, not when. If no inversion is used, a more neutral pattern is: He had no sooner completed his first novel than he fell seriously ill. In exam contexts, however, the version with initial no sooner and inversion is standard and preferred.

Step-by-Step Solution:
Step 1: Recall the structure: No sooner + auxiliary (had) + subject (he) + past participle (completed) + than + clause.Step 2: Examine Option B: No sooner had he completed his first novel than he fell seriously ill. It follows the exact pattern and uses than correctly.Step 3: Examine Option A: No sooner he had completed his first novel than he fell seriously ill. This does not invert had and he, so it violates the inversion rule.Step 4: Examine Option C: No sooner he completed his first novel than he fell seriously ill. Here again there is no inversion and no auxiliary had, making the structure non-standard.Step 5: Examine Option D: No sooner had he completed his first novel when he fell seriously ill. This option uses when instead of than, which is incorrect with no sooner.Step 6: Conclude that Option B is the only option that follows both inversion and the correct correlative pair no sooner ... than.

Verification / Alternative check:
Compare with similar examples: No sooner had the train left than it began to rain, or No sooner had I reached home than the phone rang. In each case, had precedes the subject, and than introduces the second action. You never see No sooner the train had left than or No sooner had the train left when in standard exam English. This confirms that Option B is the model answer.

Why Other Options Are Wrong:

• Option A: No sooner he had completed his first novel than he fell seriously ill lacks inversion; had should come before he.
• Option C: No sooner he completed his first novel than he fell seriously ill omits the auxiliary had and still lacks inversion, making it grammatically weak in formal usage.
• Option D: No sooner had he completed his first novel when he fell seriously ill uses when instead of than, which breaks the standard correlative pairing.

Common Pitfalls:
Students often remember the phrase no sooner but forget that when it begins a sentence, English typically uses an inverted word order similar to questions. They may also confuse the correct partner than with when, because in everyday speech people sometimes use less formal patterns. In exams, markers expect the textbook structure, so it is important to memorise the full frame: No sooner had subject past participle than clause. Practising with multiple examples will help fix this pattern in memory and reduce errors.

Final Answer:
The correct and most grammatically accurate sentence is No sooner had he completed his first novel than he fell seriously ill.