The correct option is C c
Let’s examine the sentence: “He told us that he has not read the book.” The sentence is in the form of indirect speech. Indirect speech is also known as Reported Speech, Indirect Narration or Indirect Discourse. In grammar, when you report someone else's statement in your own words without any change in the meaning of the statement is called indirect speech. In indirect speech, the simple past tense usually changes to the past perfect. The formula for the past perfect tense is had + [past participle].
When we see the sentence against these conditions, it should use verbs of past perfect tense after in the clause ‘that.’ This is because the incident happened in the past and he did not read the book in the past. So, this indirect speech would use past perfect tense and ‘has’ should be replaced by ‘had’ here. So, Option C indicates the part of the sentence with error and is the correct answer.