Answer: Ability.
Step-by-step explanation: When we want to form the plural of a word that ends in "y" there are two possibilities: if the letter before the "y" is a vowel, then we form the plural by only adding an "s" at the end, for example "boy" has an "o" before the "y" so the plural is "boys." If the word has a consonant before the "y" then we replace the "y" with "ies" for example "baby" has a "b" before the "y" so the plural is "babies." In the given words, the one that has a consonant before the "y" is "ability" which plural form is "abilities."