In each of the following questions, find out which part of the sentence has an error. The sentence is divided into three parts labelled A, B and C, and part D suggests that there is no error. Identify the part that contains a grammatical error in the sentence: Mohan is one of those boys who has expressed willingness for joining the educational tour.

Introduction / Context:
This question examines whether you can correctly handle subject verb agreement in sentences containing phrases like one of those who. Such constructions are common in competitive exams because they mix singular and plural nouns and require careful attention. You must decide which part of the sentence describing Mohan and the educational tour contains a grammatical error, if any.

Given Data / Assumptions:
- Complete sentence: Mohan is one of those boys who has expressed willingness for joining the educational tour. - Part A: Mohan is one of those boys. - Part B: who has expressed. - Part C: willingness for joining the educational tour. - Part D: The sentence is grammatically correct; no error.

Concept / Approach:
In sentences of the pattern one of those boys who, the relative pronoun who refers back to those boys, which is plural, not to one, which is singular. Therefore, the verb following who should also be plural. The correct form is who have expressed, not who has expressed. This is a classic point of subject verb agreement tested in examinations. The rest of the sentence, including the expression willingness for joining the educational tour, is acceptable in form and meaning.

Step-by-Step Solution:
Step 1: Examine part A, Mohan is one of those boys. This part correctly states that Mohan belongs to a larger group of boys. Step 2: Examine part B, who has expressed. The antecedent of who is those boys, a plural noun phrase. Therefore, the verb after who should be have expressed, not has expressed. Step 3: Examine part C, willingness for joining the educational tour. This phrase is grammatically acceptable and communicates that the boys are willing to join the tour. Step 4: Since the only error is the singular verb has with the plural antecedent, part B is the portion that must be marked as wrong.

Verification / Alternative check:
Rewrite the sentence correctly: Mohan is one of those boys who have expressed willingness for joining the educational tour. Now, the verb have properly agrees with the plural subject those boys referred to by who. The sentence becomes grammatically correct and clear in meaning. Because no other part of the sentence requires modification, this confirms that the error lies only in part B, and selecting that part as the answer is justified.

Why Other Options Are Wrong:
Part A is correct because it clearly sets up the idea that Mohan belongs to a group. Part C is correct since the phrase willingness for joining the educational tour conveys the intended meaning and is grammatically acceptable. Part D is incorrect because the sentence does have an error in subject verb agreement in part B.

Common Pitfalls:
Many students wrongly make the verb agree with one instead of those boys. This happens because one is closer to the verb and seems like the subject at first glance. To avoid this trap, always identify the noun to which the relative pronoun refers. In one of those boys who, the word who refers back to boys, which is plural. Practising similar patterns, such as one of the students who have applied, will help you internalise the correct agreement and avoid losing marks on such questions.

Final Answer:
The error is in the part who has expressed, which should be changed to who have expressed.