This one’s a bit easier, but still tricky. It’s another case of contraction…who’s.

Who’s = who is/has   It’s a contraction that combines the words who & is or who & has.

  Who’s going to the party later?
  Who is going to the party later?

  Who’s been to the zoo?
  Who has been to the zoo?

Whose shows ownership - Whose bag is this?