Hidden Mate

The best move for white is Bf7+ which leads to mate in $4$ .

Hence, the answer is $\boxed{\text{Bishop in b3}}$ .

There is actually a checkmate, found by Freddie Hand.

1.Bf7+! now black has three possiblities:

1...e3 2.Qa8+ Rd5 3.Qxd5#

1...Be3 2.Bxh5+! Rxh5 3.Qf8+ Rf5 4.Qxf5#

1...Re3 2.Qb2 Re2 (forced, to stop 3.Qg2#) 3.Bxh5+ Rxh5 4.Qf6+ Rf5 5.Qxf5#.

To be specific, the best move is to move the bishop to E6 since this will put you in an attacking position by checking the king and threatening the rook at same time

The best move is Bishop in E6 as this leads to checkmate. The white queen offers a check and the king is put of from moving to f2 and f4. Black can only move room to E3. If the white queen takes the rook, this issues a checkmate as the knight on d1 prevents the queen from being taken

Define what you mean by optimal because white is two pieces up, so it's clear that a lot of moves win from this position. Instead you should say 'find the mate in 4' (assuming there's only one) or 'find the first move to a mate in 4'.

Moving the Bishop in b3 will reveal the influence of the queen on the black king which leads to mate.

If we move the bishop, we can give a check . Thats why!

Always look for checks, in the move Bf7, the white queen checks the black king while the bishop is attacking the pawn and can capture the pawn anytime for another check. After Bf7, the only move for black is to defend the king. This is a clear win for white and black must resign before the checkmate comes.