The correct answer is: A. FDMA/FDD and TDMA/FDD.
FDMA (Frequency Division Multiple Access) is a multiple access technique in which the available frequency spectrum is divided into a number of sub-bands, each of which is assigned to a single user. TDMA (Time Division Multiple Access) is a multiple access technique in which the available time slot is divided into a number of time slots, each of which is assigned to a single user. FDD (Frequency Division Duplexing) is a duplexing technique in which the uplink and downlink frequencies are separated.
Second generation cellular systems use both FDMA and TDMA to provide multiple access to the radio spectrum. FDMA is used to divide the available frequency spectrum into a number of sub-bands, each of which is assigned to a single user. TDMA is used to divide the available time slot into a number of time slots, each of which is assigned to a single user. FDD is used to separate the uplink and downlink frequencies. This allows for more efficient use of the radio spectrum.
A. FDMA/FDD and TDMA/FDD is the correct answer because it is the only option that includes both FDMA and TDMA. B. TDMA/FDD and CDMA/FDD is incorrect because it does not include FDMA. C. FDMA/FDD and CDMA/FDD is incorrect because it does not include TDMA. D. FDMA/FDD only is incorrect because it does not include TDMA.