The Teacher replies: It is the ear of the ear, the mind of the mind, the speech of speech, the breath of breath, and the eye of the eye. When freed (from the senses) the wise, on departing from this world, become immortal.

In plain words: The teacher answers that it is the power behind the ear, mind, speech, breath and eye, the thing that makes them work at all. The wise who see this and stop identifying with the senses become immortal when they leave this world.