Protect your piece
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In this level, your opponent is attacking one of your pieces. You must protect that piece.
Protecting a piece means that you move another piece in a way that it is able to recapture if the attacked piece is captured.
This works best if the piece being attacked is less valuable than the attacking piece, otherwise protecting the piece would not avoid material losses (although it could still minimize it, if no better option is available).
Black's bishop on c5 is attacking White's unprotected knight on d4.
Running away with the knight is not possible, as it would leave the white king on g1 in check (the knight is "pinned").
The correct move is 1.c3, protecting the knight on d4. If Black captures with Bxd4, White can now recapture with cxd4.
The white rook on b1 attacks Black's unprotected bishop on a5.
Running away with the bishop allows White to capture the other bishop on b7 with Rxb7.
Therefore, you have to protect your bishop with your pawn and play 1...a5. Now Rxb4 is no longer a good move for white, as it would now lose an exchange (rook for bishop or knight) after axb4.
Advanced question: Could White win one of the bishops now with 2.a3?
Answer: No, Black could defend against that with the counterattack 2...Be4!