Which mortal does Athena disguise herself as to advise Telemachus? Mentor. In the Odyssey, Mentor is an elderly Ithacan friend of Odysseus whom the king entrusted with his household and son before leaving for Troy.
Athena adopts Mentor's appearance when Telemachus needs guidance during his father's long absence. Speaking through the trusted older man, she encourages Telemachus to confront the suitors consuming Odysseus's estate and to search Pylos and Sparta for news of his father. This begins the poem's opening Telemachy, the first four books centered on Telemachus's growth.
A frequent source of confusion is Mentes. Athena first visits Telemachus disguised as Mentes, one of Odysseus's guest-friends, in Book 1. She later appears in Mentor's form, so both names are associated with her interventions. The question's wording points specifically to Mentor.
Mentor's name later became the ordinary English word for a trusted adviser or teacher. That modern meaning grew from the character's role as Telemachus's guardian and guide, especially through later retellings and François Fénelon's influential 1699 book.