The correct answer is: A. GSM
UMTS (Universal Mobile Telecommunications System) is a third-generation (3G) cellular network technology that is designed to provide high-speed data and voice services. It is not backward compatible with GSM (Global System for Mobile Communications), which is a second-generation (2G) cellular network technology. This means that UMTS phones cannot be used on GSM networks, and vice versa.
IS-136 and IS-95 are two other 2G cellular network technologies. They are also not backward compatible with UMTS.
GPRS (General Packet Radio Service) is a packet-switched data service that is used on GSM networks. It is not a 3G technology, but it can be used to provide some 3G-like services, such as mobile internet access.
In conclusion, UMTS is not backward compatible with GSM. This means that UMTS phones cannot be used on GSM networks, and vice versa.