Their _____ house is by the side of the road.

Analyze the question: The sentence needs a word that shows possession: whose house is by the road? It belongs to "their father-in-law," so we need the possessive form of a compound noun.

Concept: With compound nouns such as father-in-law, mother-in-law, and brother-in-law, the possessive 's goes at the end of the whole compound word.

• Singular possessive: father-in-law's
• Plural: fathers-in-law (the plural s goes on the main noun, "father")
• Plural possessive: fathers-in-law's

Checking each option:

• A. father-in-law's ✔ The apostrophe-s is at the end of the compound, so it correctly shows that the house belongs to one father-in-law.
• B. father's-in-law ✘ The apostrophe is placed in the middle of the compound, which is wrong.
• C. fathers-in-law ✘ This is plural but not possessive. It would need 's to modify "house."
• D. father-in-laws ✘ The plural s is placed wrongly at the end, and there is no possessive marker.

Answer: The sentence reads: "Their father-in-law's house is by the side of the road." This is Option A.

Tip: For compound nouns, make the plural first (on the main word: fathers-in-law), and add the possessive 's at the very end. When the owner is a single person, just put 's at the end of the whole word: mother-in-law's, brother-in-law's.