It's "To the best of my ability"
But which is (more) correct:
- to the best of their ability
- to the best of their abilities
I'm finding both options on linguee.com about equally often, so I guess whatever I choose will not be ridiculously wrong.
But I am still curious if only one is correct according to some rule in the English language.
Best Answer
Let's be clear, first of all, that plural groups can have singular attributes ("The peasants' mood was rebellious"). And the answer in the page Edwin linked talks about the "distributive plural" ("It's time for all the actors to get their costume on"). By this principle, "to the best of their ability" is understood to mean "each person, to the best of his or her individual ability."
But it seems to me, there's one more opportunity for confusion here, because the word is "ability." We often speak of distinct individual abilities, like talents ("Everyone should leverage their skills and abilities"). This usage might lead us to consider choosing the plural here.
But I would argue that the phrase "to the best of [one's] ability" is distinct from this usage. It paraphrases to "the best that one is able"; the word ability in this phrase does not refer to a distinct skill but to one's overall capacity. It's a superlative; "Dance to the best of your ability" means "dance your 'bestest.'" It speaks of a cap on maximum effort and attainment. It would be silly to say to one person "do your best" and, to a group, "do your bests." So no, I would argue that each person has only one "best of [his or her] ability," and that meaning of "ability" is the same for each person (even if the capacity is not), so we should use the singular "ability" and let the distributive plural take care of the rest.