Four words have been given, out of which three are alike in some manner and one is different. Select the word that is different.

Mistress

Guide - A guide is a person who helps people for unknown locations

Instructor -An Instructor is just like a teacher who instruct of just about anything

Mentor - A mentor gives help to less experienced person

Mistress - A married man 's mistress is a woman who is not his wife

Expect Mistress,other are almost synonyms of each other