Concept:The possessive form of a compound noun is made by adding 's to the last word.Explanation:"Father-in-law" is a singular compound noun.To show that the house belongs to him, add 's to the end of the compound.Thus, "father-in-law's" is the correct possessive form.The other options incorrectly place the apostrophe or use wrong plural forms.Therefore, the best word to complete the sentence is "father-in-law's".Answer:A. father-in-law's