Final answer:
TRICARE enrollees whose sponsor initially enlisted or was commissioned before January 1, 2018, fall into Group A beneficiary category.
Step-by-step explanation:
TRICARE enrollees whose sponsor initially enlisted or was commissioned before January 1, 2018, fall into Group A beneficiary category. Group A includes all active duty service members and their families, as well as retired service members and their families. These beneficiaries have different cost-sharing requirements compared to other groups.