We use who when referring to people or when we want to know the person.
• The person who answered the phone was very helpful.
• Who ate all the chocolates?
We use which to refer to a thing or an idea, and to ask about choices.
• My car, which is 20 years old, isn't worth much.
• Which size would you like, small, medium or large?
We use that for both a person and a thing/idea.
• I'm talking about the person that I saw yesterday.
• This is the style that I want to use.
Whose is a possessive pronoun used to refer to ownership.
• Whose dictionary is this?
• There's the girl whose car was stolen.
When who is the object, whom, with a preposition, can be used instead, but it is formal and rather old-fashioned. In modern speech, we use who, or we leave out the pronoun.
• You are referring to a person who no longer works here.
• The person to whom you are referring no longer works here.
• The person (who) you are you referring to no longer works here.
Where (relative adverb) refers to places and locations.
• Where is the station please?
• That's where I spent my childhood.