Money Play. How should Red play 21?
Red needs his checkers working for him. Behind in the race and with White having 12 checkers in the attack zone this is not the time to split the rear checkers.
Red needs to work on improving his forward position and so 15/13, 6/5 can be discarded. It is safe but that is not the priority here.
The correct play by a long, long way is 13/10. The builder acts on the 4-pt and if the checker on the 15-pt is hit, White must sacrifice his mid-point to do so. All plays other than 13/10 are errors and in fact anything other than 15/14, 13/11 is at least a single blunder.