It sounds like you are asking which hitch ball is correct for the Curt TruTrack Weight Distribution Hitch # C17499 you were looking at.
This hitch uses a hitch ball with a 1-1/4" shank so you will need either the 2" hitch ball # A-90 or 2-5/16" hitch ball # 19286 to match the coupler of your trailer. These hitch balls have a shank that is 2-5/8" long or longer which is perfect for a weight distribution hitch.
Keep in mind that this hitch is designed for trailers that have a total tongue... view full answer...