a folder created in parent domain called a$.$local and shared with permissions $($R&W$)$ for particulare security group. however, users wanted are not found from child domain x$.$a$.$local

what can be the problem?

$1.$ security group not in right OU$?$

$2.$ or$,$ users are locked?

$3.$ or$,$ security group created in global group?

$4.$ or $,$ security group is created in Universal group $?$

what is the right answer and why?