In this situation, can the White king take the Black rook? Normally this could not occur since the rook is guarded, but the Black knight is pinned to its king so that it is unable to carry out a threat on the White king.
The king can't capture the rook, even if the guard (knight) is pinned to the king. The knight controls that square where the rook stands. If you take the rook (assuming it's legal), then it'll be your opponent's turn--knight will capture your king. Your king will be captured first--game over.