The correct answer is: A. GMSK
GMSK (Gaussian minimum-shift keying) is a modulation technique used in digital cellular networks, such as GSM. It is a type of frequency-shift keying (FSK) modulation in which the frequency of the carrier signal is modulated by a Gaussian pulse. This results in a signal with a very low out-of-band power, which makes it suitable for use in mobile networks where there is a lot of interference.
BPSK (binary phase-shift keying) is a modulation technique in which the phase of a carrier signal is modulated between two states, 0 and 1. This results in a signal with a constant amplitude, but with a varying phase. BPSK is a simple and robust modulation technique, but it is not very efficient in terms of bandwidth.
QPSK (quadrature phase-shift keying) is a modulation technique in which the phase of a carrier signal is modulated between four states, 0, 90, 180, and 270 degrees. This results in a signal with a constant amplitude, but with a varying phase. QPSK is more efficient than BPSK in terms of bandwidth, but it is also more complex to implement.
GFSK (Gaussian frequency-shift keying) is a modulation technique in which the frequency of a carrier signal is modulated by a Gaussian pulse. This results in a signal with a very low out-of-band power, which makes it suitable for use in mobile networks where there is a lot of interference. GFSK is similar to GMSK, but it uses a different type of pulse.
In conclusion, the correct answer is: A. GMSK.