Starting from White’s king and working outward, you find a pin of the bishop on d3. One question to ask is whether you can take it; another, however, is what it is supposed to be protecting that might now be vulnerable because of the bishop’s paralysis. Here the bishop helps protect the pawns on c4 and e4. The pawn of interest is the one on c4, as Black already attacks it twice. Because of the pin on the bishop, the pawn is protected only once. Black can take it next move.