Answer
When considering mass, Ca+ and Ar+ are the closest to K+.
However, when considering ionization energy, the first IE of Ca+ is closest to K+, so Ca+ is our best choice for a replacement.
Work Step by Step
A substitute for K+ would need to have a 1+ charge and have other similar physical properties, like: mass and atomic radius.
Na+ and Rb+ wouldn't be considered good replacements for K+ as their radii are a lot smaller and larger, respectively.
