Error spotting: identify whether the sentence He has made a mistake of which I am certain contains any grammatical error.

Introduction / Context:
Error spotting questions ask you to look at different parts of a sentence and decide whether any part contains a grammatical or usage error. Here the full sentence is He has made a mistake of which I am certain. The options correspond to segments of the sentence, and one option stands for No error. Our task is to confirm whether the sentence is grammatically correct as it stands.

Given Data / Assumptions:

The complete sentence is He has made a mistake of which I am certain.
Parts are divided as A He has made a mistake, B of which, C I am certain, and D No error.
Standard modern English usage is the benchmark for correctness.

Concept / Approach:
We must check the tense, verb form, preposition usage, and clause structure. The present perfect form has made is appropriate for a past action with present relevance. The phrase of which introduces a relative clause I am certain, which describes the noun mistake. There is nothing unusual in this structure; it is similar to expressions such as a fact of which I am aware or a decision of which we are proud. Therefore we need to see whether any segment violates grammar rules or natural usage.

Step-by-Step Solution:
Step 1: Examine Part A He has made a mistake. The subject he and the present perfect verb has made agree correctly. Step 2: Examine Part B of which. This preposition plus relative pronoun combination correctly links back to the noun mistake. Step 3: Examine Part C I am certain. This is a complete clause with subject I and verb am and is grammatically sound. Step 4: Combine the parts mentally to read He has made a mistake of which I am certain and check whether the full sentence is clear and idiomatic. Step 5: Conclude that no part contains an error and therefore the correct choice is No error.

Verification / Alternative check:
We can paraphrase the sentence as I am certain that he has made a mistake. This alternative structure shows that the original sentence simply uses a more formal relative clause. Both sentences convey the same meaning and are grammatically correct. Since the paraphrase is acceptable and there is no mismatch of tense or agreement, it confirms that the original sentence is error free.

Why Other Options Are Wrong:
Option A claims that He has made a mistake is wrong, but this section has correct word order and tense.
Option B suggests that of which is wrong, yet this is a standard way to introduce a relative clause that modifies mistake in formal English.
Option C suggests that I am certain is wrong, but this clause is a correct independent clause that expresses the speaker s certainty.
Option E claims that the whole sentence must be rewritten; this is incorrect because the sentence is already precise and grammatical.

Common Pitfalls:
Candidates sometimes distrust formal patterns like of which I am certain because they sound old fashioned, and they assume that anything that sounds formal must be wrong in exam questions. Another frequent error is to think that every error spotting question must contain an error, when in reality many exam papers deliberately include fully correct sentences to test careful reading. The safe strategy is to analyse each part for actual grammatical mistakes rather than guessing based on style alone.

Final Answer:
There is no grammatical error in the sentence, so the correct choice is No error.