grammatical number - Plural of "daughter in law"

Words that are pluralized in the middle?

I was asked to write the plural of daughter in law in an aptitude exam. I wrote it as daughters in law. Please tell me the right answer.

Answer

You are right.
When pluralizing words like "-in-law" or "-in-chief", The noun is pluralized, not the modifiers following. Hence:

"Fathers-in-law" not "Father-in-laws"

and

"Commanders-in-chief" not "Commander-in-chiefs"