_________ is the correct compound noun in the plural form.
(a) Brothers-in-law
(b) Brother-in-laws
(c) Brothers-ins-laws
(d) Brethren-in-law
Answer options :

The correct answer is Option D ((a) only).

Key Points:
• Core Language Concept Tested: The question primarily tests the candidate's understanding of Noun Numbers, specifically the rules for forming the plural of hyphenated compound nouns.
• Why the Correct Option is Right: The official answer key marks option 4 as correct, identifying '(a) Brothers-in-law' as the correct form. This is absolutely accurate. The standard rule for pluralizing a hyphenated compound noun containing a noun and a prepositional phrase is to add the plural suffix '-s' to the principal or head word. Here, the primary noun is 'brother', so it becomes 'brothers'. The prepositional phrase 'in-law' remains unchanged.
• Common Exam Traps and Pitfalls: A common mistake candidates make is treating the entire compound noun as a single un-hyphenated word and attaching the 's' at the very end, resulting in the incorrect 'brother-in-laws' (b). This is how it is often incorrectly spoken in informal English, making it an excellent trap.
• Elimination of Incorrect Options: Option (c) 'Brothers-ins-laws' absurdly pluralizes every element. Option (d) 'Brethren-in-law' uses an archaic plural of brother (brethren), which is used for members of a religious order or society, not for legal family relationships. Thus, by a process of systematic elimination based on strict grammar rules, we arrive safely at the correct choice.
• Preparation Strategy: Memorize plural formations for common compound nouns (e.g., commanders-in-chief, passers-by, mothers-in-law).
• Key Takeaway: Remember: To pluralize a compound noun like 'brother-in-law', add 's' to the principal word, making it 'brothers-in-law'.