I need to know that which solution is used to keep the pH meter sensor wet. Some people say that KCl is used but which concentration and pH are better?
The most commonly used solution to store a wet pH electrode in is 4 M KCl.
Read the manual that came with the exact pH electrode that you have for the correct answer. You may also be able to download the information from the manufacturer's website too.
3 M KCl, i.e. about saturated KCl, is used for the conventional electrodes. Some electrodes may have different requirements; it is best to refer to the user manual.