What episode was it when rachel said that job a transponder?

You're likely thinking of the episode where Rachel is interviewing for a job and says "transponder" instead of "responder". This happens in Season 5, Episode 16, "The One With The Cop."

Rachel is interviewing for a job at Bloomingdale's and trying to impress the interviewer by talking about her experience working at Ralph Lauren. When asked about how she handled a difficult customer, she mixes up the words and says, "I was like a transponder, I just listened."