Chess has an informal price list, measured in pawns: a knight or bishop is worth about 3, a rook about 5, and the queen about 9. The king has no price because losing it ends the game. These numbers aren't official rules, they're centuries of accumulated experience about how much work each piece typically does.
The price list is what turns "should I take?" into arithmetic. Trading a knight (3) for a rook (5) earns you 2 points of material; giving a rook (5) for a bishop (3) costs you the same. When two captures are available at once, the list tells you instantly which one is bigger.
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Two black pieces hang at once: the queen on d8 (to the knight) and the rook on b7 (to the bishop).
Two captures are available. Take the more valuable piece.
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