In the following question, a sentence is divided into four parts. Identify the part that contains an error in standard English usage, or select 4 for No error: The reason for / his failure is because / he did not study at all. / No error.

Introduction / Context:
This grammar question checks the correct way to express reason in English. The sentence as given contains a redundancy that is common in everyday speech but is considered incorrect in standard written English. Examinations often test this structure because many speakers do not notice the extra word. Learning the precise pattern helps candidates write more concise and accurate sentences, which is essential for formal communication and competitive exams.

Given Data / Assumptions:
- The sentence is: The reason for his failure is because he did not study at all.
- It is split into four parts: (1) The reason for, (2) his failure is because, (3) he did not study at all, (4) No error.
- We have to identify the part that contains the grammatical error.
- The intended sense is to state why he failed.

Concept / Approach:
The main concept is redundancy in expressions of reason. The phrase the reason is already indicates cause, so it should be followed by a clause introduced by that or by a simple clause, not by because. The forms The reason is that he did not study at all or He failed because he did not study at all are both correct, but combining reason with is because creates an unnecessary duplicate expression of cause. This structure is considered incorrect in standard usage and is often highlighted in grammar textbooks.

Step-by-Step Solution:
Step 1: Look at each part separately. Part 1, The reason for, is a correct beginning of a noun phrase.Step 2: Part 2, his failure is because, joins the subject phrase to a clause of reason using is because.Step 3: Recognise that the combination reason and because is redundant. We should either say The reason for his failure is that he did not study at all, or He failed because he did not study at all.Step 4: Therefore, the error is located in part 2, where because is incorrectly used after the word reason.Step 5: The corrected sentence is The reason for his failure is that he did not study at all.

Verification / Alternative check:
We can cross check by comparing similar sentences. We say The reason for the delay is that the train arrived late, not The reason for the delay is because the train arrived late. Also, we can restate the idea without the noun reason: He failed because he did not study at all, which shows that because alone is enough to introduce the clause of cause. Thus, including both reason and because together is not stylistically acceptable in formal English.

Why Other Options Are Wrong:
- Part 1, The reason for, is fine and is often used in such constructions.
- Part 3, he did not study at all, correctly explains the cause and contains no grammatical error.
- Part 4, No error, is not correct because we have clearly located an error in part 2.

Common Pitfalls:
Many learners copy patterns from informal speech where is because after reason may occur. Another pitfall is focusing only on verb forms and ignoring logical redundancy. When checking for errors, always consider whether any word or phrase repeats the same idea unnecessarily. Remember that after the phrase the reason is, the conjunction that is the standard choice, and using because directly after reason should be avoided in precise written English.

Final Answer:
The incorrect segment is part 2, since the phrase should be his failure is that he did not study at all, not his failure is because he did not study at all.