In this simple sentence correctness question, decide which option correctly identifies an error in the sentence "He could not find an answer", or whether the sentence has no error.

Introduction / Context:
This question evaluates basic sentence structure, modal verbs, and article usage in English. The sentence is "He could not find an answer." Candidates must check if any segment of the sentence is grammatically unacceptable or unnatural. Since the structure is quite common in everyday English, recognising it as correct is part of developing confidence with simple yet frequently used patterns.

Given Data / Assumptions:

• The sentence expresses a failure to find a solution.
• Segment (a) is "He could not".
• Segment (b) is "find".
• Segment (c) is "an answer".
• Segment (d) is "No error".
• We assume standard modern English usage with full forms rather than contractions.

Concept / Approach:
"Could not" is a correct combination of the modal verb "could" with the negative particle "not", used to express inability or failure in the past. The verb "find" appropriately follows the modal without "to". The noun phrase "an answer" uses the indefinite article "an" correctly because "answer" begins with a vowel sound. Putting the parts together, the sentence "He could not find an answer" is grammatically complete and meaningful. There is no error in any part of the sentence.

Step-by-Step Solution:
Step 1: Examine segment (a) "He could not". It has a correct subject "He" and a negative modal phrase "could not" for past inability. Step 2: Examine segment (b) "find". After a modal verb, the verb always appears in its base form, which is satisfied here. Step 3: Examine segment (c) "an answer". The article "an" is correctly used before a noun beginning with a vowel sound, and "answer" fits this pattern. Step 4: Combine the segments to read "He could not find an answer." The sentence is clear and fully grammatical. Step 5: Since no segment shows any problem in tense, agreement, or article usage, the correct choice is "No error".

Verification / Alternative check:
For further confidence, compare with similar sentences: "She could not solve the problem", "They could not reach the station in time", and "I could not remember his name". All these follow the same pattern: subject plus "could not" plus base verb plus object. No one would consider these sentences erroneous. The given sentence fits exactly into this model and therefore must also be correct. This verification confirms that there is no hidden mistake.

Why Other Options Are Wrong:
Option (a) "He could not" is a proper way to express inability in the past. Option (b) "find" is correctly used after the modal "could". Option (c) "an answer" is a correct noun phrase with the right article. Since all three segments are correct, option (d) "No error" is the only accurate choice.

Common Pitfalls:
Sometimes learners mistrust short, simple sentences and try to find errors by overanalysing. They may think a more complex tense is required or that "could not be able to find" would sound better, but those changes only make the sentence heavier and less natural. It is important in exams to recognise when a sentence is already correct and not to introduce unnecessary complications. Trusting well known basic structures is an important exam skill.

Final Answer:
The sentence "He could not find an answer" has no grammatical error. The correct answer is "No error".