In the following sentence, identify the part that has an error: I had invited (A) / all my sister-in-laws (B) / to my son's birthday party. (C) / No error (D).

Introduction / Context:
This error spotting question focuses on the correct plural form of a compound noun, specifically sister-in-law. The sentence describes someone inviting multiple relatives to a birthday party. The grammatical issue is with how the plural is formed when a noun is combined with prepositions like in and other words such as sister and law.

Given Data / Assumptions:

• Sentence parts: I had invited (A) / all my sister-in-laws (B) / to my son's birthday party. (C) / No error (D).
• Part A: I had invited.
• Part B: all my sister-in-laws.
• Part C: to my son's birthday party.
• The speaker is talking about more than one sister-in-law.

Concept / Approach:
In English, many compound nouns form the plural by adding s to the main noun, not to the entire phrase. In the expression sister-in-law, the core noun is sister, and in-law is a postmodifier. Therefore, the plural is sisters-in-law, not sister-in-laws. The same pattern appears in brothers-in-law, mothers-in-law, and so on. Recognising where to put the plural s is crucial for correct usage in formal writing and exams.

Step-by-Step Solution:
Step 1: Identify the compound noun: sister-in-law. Step 2: Determine that the speaker is referring to more than one such relative by using all my, which signals plurality. Step 3: Recall the rule for compounds like this: add s to the main noun in the compound, not at the end of the entire phrase. Step 4: Apply the rule: sister becomes sisters while in-law remains unchanged, giving sisters-in-law. Step 5: Examine part B: all my sister-in-laws. This incorrectly places the s at the end of law instead of on sister. Step 6: Check parts A and C: I had invited and to my son's birthday party are grammatically correct and natural. Step 7: Conclude that the error is in part B.

Verification / Alternative check:
Look at similar English expressions: She met her mothers-in-law, They are my brothers-in-law, and We spoke to several editors-in-chief. In each case, the main noun is pluralised while the following elements remain singular. No reliable grammar book or dictionary lists sister-in-laws as the standard plural; instead, they all show sisters-in-law. Rewriting the sentence as I had invited all my sisters-in-law to my son's birthday party confirms that this version sounds correct and idiomatic.

Why Other Options Are Wrong:
Part A is correct because I had invited is an appropriate use of the past perfect to describe an action completed before a later reference point. Part C is also correct; to my son's birthday party clearly indicates the purpose or destination of the invitations, and son's is correctly written as a possessive. Part D, No error, is wrong because the incorrect plural form in part B proves that the sentence does contain an error.

Common Pitfalls:
Candidates often pluralise the last word in a compound noun by habit, leading to forms like sister-in-laws, which are widespread in casual speech but not accepted in formal exams. Another pitfall is not knowing which part of a compound is the main noun. A simple check is to see which word can stand alone as the primary category; here it is sister, not law. Learning the plural patterns of common compound family terms will help you avoid such mistakes.

Final Answer:
The error is in part B, which should read all my sisters-in-law.