Q: If a carrier modulated by a digital bit stream had one of the possible phases of 0, 90, 180, and 270 degrees, then the modulation is called:

The correct answer is option 'D', QPSK (Quadrature Phase Shift Keying).

Explanation:

1. Introduction to Digital Modulation:
Digital modulation is a process of encoding digital information onto an analog carrier signal for transmission. It involves manipulating certain characteristics of the carrier signal to represent the digital data. Some common characteristics that can be manipulated are amplitude, frequency, and phase.

2. Phase Shift Keying (PSK):
Phase Shift Keying is a digital modulation technique that encodes the digital data in the phase of the carrier signal. In PSK, the carrier signal can have multiple phase states, each representing a different bit or a combination of bits.

3. BPSK (Binary Phase Shift Keying):
BPSK is a form of PSK where the carrier signal can have two phase states: 0 degrees and 180 degrees. It is the simplest form of PSK modulation, where each bit is represented by a different phase state.

4. QAM (Quadrature Amplitude Modulation):
QAM is a modulation scheme that combines both amplitude and phase modulation. In QAM, the carrier signal can have multiple phase states and amplitude levels. It is commonly used to transmit higher data rates and allows for more efficient use of the available bandwidth.

5. MSK (Minimum Shift Keying):
MSK is a form of frequency modulation that uses phase modulation to encode digital data. In MSK, the carrier signal changes its phase at the midpoint of each data bit. It is a form of continuous phase modulation and is known for its constant envelope, which makes it suitable for transmission in power-limited systems.

6. QPSK (Quadrature Phase Shift Keying):
QPSK is a form of PSK that uses four different phase states to represent the digital data. It is an extension of BPSK, where each bit is represented by a combination of two different phase states. In QPSK, the carrier signal can have phase states of 0 degrees, 90 degrees, 180 degrees, and 270 degrees.

7. Conclusion:
In the given scenario, where the carrier modulated by a digital bit stream has possible phases of 0, 90, 180, and 270 degrees, the modulation technique used is QPSK. QPSK allows for the transmission of two bits per symbol and provides better spectral efficiency compared to BPSK.