Answer:
D: mother-in-law's
Step-by-step explanation:
The correct possessive form of the compound noun mother-in-law to complete the sentence is made by adding an apostrophe and an "s" to it: mother-in-law's. Only the last word is used to show possession when it is a singular compound word. In the case of plural, as in C, only the apostrophe is added to the "s" of the last word.