"This is It" was featured in the Season Nine episode, "Follow That Egg!". It was the theme song for the 1970s comedy, One Day at a Time.
Background
Stan Marsh and Bebe Stevens are paired together by Mr. Garrison, for their assignment of taking care of an egg for a week. Stan shows little regard for the egg and throws it on the couch, choosing instead to watch TV. This song is heard on the channel he tunes to.