respond vs say

respond is not a noun while say is a noun.

Word	Noun	Adjective	Verb	Adverb
respond	No	No	Yes	No
say	Yes	No	Yes	No

As verbs, say is a hypernym of respond; that is, say is a word with a broader meaning than respond:

Other hypernyms of respond include state, tell.

respond (noun)	say (noun)
	the chance to speak

respond (verb)	say (verb)
show a response or a reaction to something	express in words
respond favorably or as hoped	report or maintain
react verbally	speak, pronounce, or utter in a certain way
	express a supposition
	give instructions to or direct somebody to do something with authority
	communicate or express nonverbally
	recite or repeat a fixed text
	indicate
	state as one's opinion or judgement; declare
	utter aloud
	have or contain a certain wording or form