In the following English grammar error spotting question, read the labelled sentence Each one of his sisters (A) / were (B) / hardworking. (C) / No Error (D) and choose which part contains the error in subject–verb agreement.

Introduction / Context:
This question targets subject–verb agreement when quantifiers like each one are used before plural nouns. The sentence describes a group of sisters and states that all of them are hardworking, but the grammatical structure focuses on each individual member. Many learners misread such sentences and choose a plural verb because they see a plural noun nearby, which is why exam setters frequently include items of this type. You must identify which part among (A), (B), (C), and (D) violates the agreement rule between the subject and the verb.

Given Data / Assumptions:

• Part (A): Each one of his sisters
• Part (B): were
• Part (C): hardworking.
• Part (D): No Error.
• The intended meaning is that every individual sister was hardworking.

Concept / Approach:
The key concept is that when a sentence begins with each, each of, every, or every one, the grammatical subject is treated as singular, even if followed by a plural noun. In this sentence, the subject is Each one, not sisters. Therefore, the verb should be singular was rather than plural were. The phrase of his sisters is a prepositional phrase that modifies one and does not change the singular nature of the subject. The predicate adjective hardworking correctly describes the subject and does not need any change. Consequently, the only error lies in the verb form in part (B), which must be changed from were to was to match the singular subject Each one.

Step-by-Step Solution:

Verification / Alternative check:
To verify, rewrite the sentence correctly: Each one of his sisters was hardworking. This sentence now respects the singular subject and uses the appropriate past tense verb form was. If you contrast this with a clearly plural subject, such as All of his sisters were hardworking, you can see that the choice of verb depends on the quantifier at the beginning. All takes a plural verb, but each one takes a singular verb. This contrast confirms that the original sentence should not use were after Each one and that part (B) must be corrected.

Why Other Options Are Wrong:
Part (A) is correct as a subject phrase; Each one of his sisters is a standard way to refer to every individual member of the group. Part (C), hardworking, is a simple predicate adjective that accurately describes the subject and does not require an article in this context. Part (D), No Error, cannot be chosen because we have identified a clear subject–verb disagreement in part (B). Therefore, option B is the only correct choice that marks the location of the grammatical error.

Common Pitfalls:
The most common pitfall is to be influenced by the nearest plural noun, here sisters, and to choose a plural verb automatically. This is known as proximity agreement, and it often misleads learners. To avoid it, always locate the true subject of the sentence, especially when quantifiers and prepositional phrases are present, and then match the verb to that subject rather than to a nearby noun. Memorising patterns like each of the students is and every one of the books was can help you internalise this rule and apply it quickly in exam situations.

Final Answer:
The verb in part (B) is incorrect, so the correct answer is B.