The Pharisees sarcastically reply to Jesus with “where” not “who” because they know very well “Who” He is talking about. Even little children in Christ know the Father (1 John 2:13), but these religious rulers of the Jews–who are responsible for leading the people into a relationship with God–do not.

1 John 2:13  I write to you, fathers, because you have known him who is from the beginning. I write to you, young men, because you have overcome the evil one. I write to you, dear children, because you have known the Father.

The only way to get to know the Father is to know Jesus, because He alone declares Him to the world (John 1:18).

John 1:18  No-one has ever seen God, but God the One and Only, who is at the Father’s side, has made him known.