Answer:
A.Millie might not be able to attend the play tonight because she still has to find someone to watch her two-year-old daughter for the night.
Explanation:
A compound adjective is a group of words that function collectively to modify a noun.
To properly hyphenate a compound adjective, there must be two or more words that acts as one idea.
Therefore, option A is correct where the compound adjective "two-year-old" is properly hyphenated.
