Best way to define Constants in Objective-C

Best way to define Constants in Objective-C

I am defining constants with proper naming convention in objective-C but
after having some search on internet, I found three different naming
conventions for defining the constants. Those of given below.
NSString *const kModel_userID;
NSString *const k_model_user_id;
NSString *const kUserId;
Please check these and help me out which is the best way convention for
constants in objective-C. Please give me valid reason if you like any
convention. Also If you have any other convention please share that too.
Thanks