In the sentence "By eight o'clock in the morning he wrote four letters to his friends.", the underlined verb phrase must match the time expression. Choose the option that best improves the verb phrase, or select No improvement if the sentence is already correct.

Introduction / Context:
This sentence improvement question tests your understanding of the past perfect tense when describing a completed action by a particular time in the past. The sentence states that by eight o'clock in the morning something had already been done. To show clearly that the action was completed before that time, the verb phrase must be chosen carefully.

Given Data / Assumptions:

• Sentence: By eight o'clock in the morning he wrote four letters to his friends.
• The underlined part is wrote four letters.
• Options: had written, had been writing, was writing, No improvement.
• The phrase by eight o'clock in the morning indicates a deadline up to which an action was completed.

Concept / Approach:
When we refer to a specific time in the past and want to emphasise that an action was completed before that time, we use the past perfect tense: had plus past participle. The structure by time plus past perfect clearly marks the completion of the action before the given time. Therefore, the correct form here is By eight o'clock in the morning he had written four letters to his friends. The simple past wrote does not clearly show this prior completion relative to another past reference point. The other options either focus on the ongoing nature of the activity or change the meaning.

Step-by-Step Solution:
Step 1: Identify the time marker by eight o'clock in the morning, which signals a completed action up to a particular time. Step 2: Recall the rule that the past perfect tense is used to indicate an action completed before a specific point in the past. Step 3: Transform wrote into past perfect had written, giving he had written four letters. Step 4: Combine with the time phrase: By eight o'clock in the morning he had written four letters to his friends. Step 5: Evaluate option A, had written, which matches this structure exactly. Step 6: Evaluate option B, had been writing, which would emphasise the ongoing nature of writing and not specifically the number of letters completed. Step 7: Evaluate option C, was writing, which would usually be used with while or when to describe an action in progress at that time, not one completed by that time.

Verification / Alternative check:
Compare the sentence with similar examples: By ten o'clock she had finished her homework, By lunchtime they had reached the city. In all such cases, the past perfect is preferred to make clear that the action was fully completed before the specified time. Using simple past in those sentences would make the time relationship less precise. The same reasoning applies to By eight o'clock in the morning he had written four letters, which clearly expresses that all four letters were already written by that time.

Why Other Options Are Wrong:
Had been writing focuses on the duration or process of writing, but does not match well with a specific count of letters already written, and it also fails to emphasise completion. Was writing suggests that he was in the middle of writing at eight o'clock, rather than having already finished four letters by that time. No improvement would leave wrote, which is grammatically possible but not the best tense to show completion before a past time reference in this formal examination context.

Common Pitfalls:
Students sometimes avoid the past perfect because they feel it is complicated, relying instead on simple past for all past actions. However, in competitive exams the past perfect is often tested through time expressions like by, before, after, and when. To avoid mistakes, remember that by plus a time in the past often calls for had plus past participle to show that something was already complete at that moment.

Final Answer:
The correct improvement is had written, giving: By eight o'clock in the morning he had written four letters to his friends.