A passed pawn's power is momentum: every step forward makes it more dangerous and more expensive to stop. The cure is the blockade, parking a piece directly on the square in front of it. A blockaded passer isn't just stopped, it becomes a wall that shelters your blockader and gets in its own army's way.
Knights are the best blockaders in chess: sitting in front of the pawn costs a knight none of its attacking power (it jumps over things anyway), and the pawn itself shields it from frontal attack. A rook on blockade duty is wasted; a knight on blockade duty is often doing its best work of the game.
Black's d-pawn wants to run. The square directly in front of it, d3, is one knight-jump away.
Stop the passer permanently. Park the knight in front of it.