… are often quite caring.

Analyze the question: The sentence needs a plural noun, because the verb is "are". So we need the correct plural of the compound noun "mother-in-law".

Explain the concept: In compound nouns formed with hyphens (noun + preposition + noun), the main word takes the plural "s", not the last word. The main word here is "mother", since a mother-in-law is a kind of mother.

Examples:

• brother-in-law → brothers-in-law
• sister-in-law → sisters-in-law
• passer-by → passers-by
• commander-in-chief → commanders-in-chief

Solution:

1. "are" shows a plural subject, so A (mother-in-law) is wrong because it is singular.
2. B (mother-in-laws) puts the "s" on the wrong part, so it is wrong.
3. C (mothers-in-laws) pluralizes both words, so it is wrong.
4. D (mothers-in-law) pluralizes only the main word "mother". This is correct.

Complete sentence: Mothers-in-law are often quite caring.

Tip: For hyphenated compounds like these, ask yourself "which word is the actual person or thing?" Pluralize that word only. The words after it (in-law, by, of-state) usually stay unchanged.