Correct use of relative pronouns
Who is used for persons only.
Which is used for animals and inanimate objects.
That can refer to both persons and things. It is not used in non-identifying relative clauses.
That is preferred in the following cases:
After adjectives in the superlative degree
- He is the most versatile writer that I have ever read. (More natural than ‘He is the most versatile writer who I have ever read.)
After the words all, same, any, none, nothing and only
- Take all that you want. (NOT Take all which you want.)
- He is the same man that he has been.
After the interrogative pronouns who and what
- Who is it that troubles you so much? (NOT Who is it who troubles you so much.)