Which of the following is not soft acid according SHAB concept?

Explanation

The HSAB (Hard and Soft Acids and Bases) concept classifies acids and bases as hard or soft. Hard acids have high charge density and less polarizability, while soft acids have low charge density and are more polarizable. K+ is a hard acid due to its small size and high charge density, making it different from the other options, which are soft acids. Cu+, Ag+, and Au+ are soft acids because they have lower charge density and greater polarizability, allowing them to form stable complexes with soft bases.