Sentence improvement (verb form): choose the best substitute for the underlined part in the sentence below. I did not go to the office because I was ill.

Introduction / Context:
This item checks your understanding of correct tense usage in a simple cause and effect sentence. The sentence describes a past action that did not happen because of a past state of health. Many learners overcomplicate such sentences by using unnecessary perfect tenses. Examinations often ask you to recognise that the simplest, most natural structure is best.

Given Data / Assumptions:

• Sentence: I did not go to the office because I was ill.
• Only the verb phrase \"was ill\" is considered for replacement.
• Options: felt, had become, had felt, no improvement.
• The speaker is talking about a past day when he or she missed work due to illness.

Concept / Approach:
In English, when both the cause and the action happen in the same general time frame in the past, a simple past tense is usually enough. \"Was ill\" is a stative description of a condition at that time. There is no need for a past perfect tense unless we want to show that one past action clearly finished before another past reference point. Here, both events are part of the same time frame, so the original is already correct.

Step-by-Step Solution:
Step 1: Identify the two clauses: \"I did not go to the office\" and \"because I was ill\". Step 2: Note that both relate to the same past day or period. There is no such time layering that requires a past perfect construction. Step 3: Evaluate \"felt\" and \"had felt\". The natural expression is \"felt ill\" or \"felt unwell\", not just \"felt\" on its own. Step 4: Evaluate \"had become\". \"Had become ill\" is grammatically possible but unnecessarily complex and slightly awkward here. Step 5: Recognise that \"was ill\" is perfectly standard, simple past describing a state, and therefore the best choice is to keep the original form.

Verification / Alternative check:
Try reading the sentence aloud with each option completed fully: \"because I felt ill\", \"because I had become ill\", \"because I had felt ill\", and \"because I was ill\". The last option sounds the simplest and most idiomatic. Native speakers commonly say, \"I did not go to work because I was ill.\" This confirms that no change is needed.

Why Other Options Are Wrong:
Option \"felt\": On its own, \"felt\" is incomplete; we expect \"felt ill\" or \"felt unwell\". Since only a single word is offered, the replacement does not work naturally in the sentence. Option \"had become\": The past perfect suggests an earlier past time before another past reference. That extra layer of time is not needed here and makes the sentence heavier than necessary. Option \"had felt\": Similar to \"had become\", it introduces an unnecessary past perfect structure and is incomplete without \"ill\" or another complement.

Common Pitfalls:
Candidates sometimes think that more complex tenses like the past perfect always sound more formal or more correct. In reality, English prefers clear and simple tense usage unless there is a strong reason to show a specific sequence of past events. Overuse of the past perfect is a frequent exam mistake.

Final Answer:
The best version of the sentence is unchanged: I did not go to the office because I was ill. Hence, the correct option is no improvement.