Find out which part of a sentence has an error.She is the teacher (A)/ whom I know (B)/ has helped my son in the examination. (C)/ No error (D)

Find out which part of a sentence has an error.

She is the teacher (A)/ whom I know (B)/ has helped my son in the examination. (C)/ No error (D)
1). A
2). B
3). C
4). D

This Question has 2 answers.

"She is the teacher who I know has helped my son in the examination."

Error Explanation: "Whom" is incorrect because it is the object form.

"Who" should be used as it is the subject of the clause "has helped my son."

2 -Whom replace to who