In the following sentence improvement question, read the labelled sentence Mother-in-laws (A) / are (B) / a nuisance. (C) / No Error (D) and choose which part (A), (B), (C) or (D) contains the grammatical error.

Introduction / Context:
This question checks your understanding of correct plural formation for compound nouns in English. The sentence is divided into three labelled parts plus a No Error option. You must determine which part contains a grammatical mistake. The sentence talks about mother-in-law as a family relationship and tries to use a plural form. Many learners are confused about where to add the plural ending in compound nouns of this type, so examiners frequently use such examples in error spotting questions.

Given Data / Assumptions:

• Part (A): Mother-in-laws
• Part (B): are
• Part (C): a nuisance.
• Part (D): No Error.
• The intended meaning is that mothers-in-law as a group are a nuisance.

Concept / Approach:
In English, when forming the plural of compound nouns like mother-in-law, the main noun is pluralised, not the entire compound. The main noun here is mother, not law. Therefore, the correct plural form is mothers-in-law, not mother-in-laws. The verb are in part (B) correctly agrees with a plural subject, and the predicate a nuisance is acceptable as a general comment about the group. Consequently, the only error is in the plural form used in part (A), where the plural marker has been attached to the wrong part of the compound noun.

Step-by-Step Solution:

Verification / Alternative check:
To verify, rewrite the whole sentence correctly: Mothers-in-law are a nuisance. This version sounds natural and follows standard grammar rules for pluralising compound nouns. If we try to retain the original form, Mother-in-laws are a nuisance, it looks and sounds odd to native speakers and does not match the pattern used in trusted dictionaries and grammar books. Because the verb are agrees with a plural subject and the phrase a nuisance is standard in such comments, there is no need to change parts (B) or (C). All evidence points to part (A) as the only incorrect segment.

Why Other Options Are Wrong:
Option B is not the error because are correctly matches the plural subject mothers-in-law. Changing it to is would create a subject verb disagreement, so are is correct. Option C is also fine, because the phrase a nuisance is a normal way to describe something or someone that causes irritation or trouble. Option D, No Error, cannot be chosen because we have identified a clear, rule based mistake in part (A). Therefore, the only reasonable choice is option A, which marks the compound noun with the incorrect plural form.

Common Pitfalls:
Students often misplace the plural ending in compound nouns. They may assume that the plural marker must come at the very end of the word, which leads them to forms like mother-in-laws instead of mothers-in-law. Another common issue is overthinking the article a in the predicate and mistakenly trying to change it, even though it is correct for generic statements. To avoid errors, remember that in such compounds, the central noun carries the plural, while the rest of the compound remains unchanged. Practising with similar examples like brothers-in-law, daughters-in-law, and sons-in-law can help fix the pattern in memory.

Final Answer:
The grammatical error is in part (A), so the correct answer is A.