Civilians are citizens of West Ardougne who can be found in the northern section of the city, just east of the Underground Pass. When talked to, they will complain that they are busy because of the rat infestation, and say that they need a cat.
Players can sell regular, overgrown, wily and lazy cats to civilians for 100 death runes each, or 200 after completing the easy Ardougne Tasks. Note that talking to them with a cat in your inventory and proceeding through the dialogue will mean losing your cat PERMANENTLY, you cannot get it back, although you can buy a replacement from Gertrude. Note that they will not accept kittens or the Witch's cat from the Underground Pass.
Note that you can proceed through the dialogue and leave the conversation early, keeping your cat and making progress in the Ardougne Tasks.