The word 'gall' matches with A) n. something that is irritating or bitter, while "galling" matches with B) adj. the quality of being irritating or bitter and "gallingly" with C) adv. in a manner that.
The word 'gall' is mainly used to refer to someone that is very rude and has an impudent behaviour, that is to say a person that does not show respect for another person. Moreover, 'galling' is an adjective, as most words that end in -ing and are not used as verbs, and it is used to describe someone that makes another person annoyed and angry. Furthermore, 'gallingly' means "in a matter that makes someone else feel irritated" and it also derives from the word 'gull'. 'Gallingly' is an adverb, as most words that end in -ly.