The best move. Usually, when one is playing against an isolated pawn, it is a good idea to block it. In this case, the plan would be to block with a knight on d4. After ...Qxc5, Black increases control over the d4-square.
After 19...Nxc5 Black abandons the d4-square, and White would be slightly better.
There was nothing wrong capturing with the rook, but this is even stronger. With the knight coming to c4 White has to be slightly more careful.
Unity Chess Club
Anatoly Karpov Zoltan Ribli Amsterdam 1980 White to move
Black is suffering some pressure on the queenside and Karpov finds a plan to immobilize his opponent!
The best move! Black was intending to play a6! For instance after 19.Kf1?! Black can play 19...a6! 20.Rxb6 20.Rb3 b5 and Black has managed to equalize. Rxb6 Bxb6.
This is the key. The pawn on b2 was hanging after the trade of rooks!
Black has played very passively, and now Karpov simply improves his position choking his opponent.
24.Bc5 Ke8 25.d4 Kd7 26.e4 e6 27.b4 Kc8 28.d5 exd5 29.exd5 Rd7 30.d6 Rd8 31.Kg2 Kd7 32.Re2 Kc8 33.Re7 Rd7 34.Ra2 a5 35.Rc2
The best move. White opens the a-file for the rook and the knight on c3 is threatening to jump to e4, which forces Black to play ...d5