*Question E2E03:
To what does the "4" in FT4 refer?*  

*Correct Answer is  B: Four-tone continuous-phase frequency shift keying*  

### AI Response: (may contain errors)  

The correct answer is B: Four-tone continuous-phase frequency shift keying.

### Explanation:

1. **Correct Answer: Four-tone continuous-phase frequency shift keying**
   - FT4 is a digital mode used in amateur radio that employs four distinct tones for encoding data. This is a form of frequency shift keying (FSK) where the phase of the signal is continuous, meaning there are no abrupt changes in phase when switching between tones. This helps in reducing bandwidth and improving signal clarity.

2. **Why the Other Choices are Incorrect:**
   - **A: Multiples of 4 bits of user information**
     - This choice is incorrect because FT4 does not specifically encode data in multiples of 4 bits. The "4" in FT4 refers to the number of tones used, not the bit structure.
   - **C: Four transmit/receive cycles per minute**
     - This choice is incorrect because FT4 does not operate on a cycle of four transmit/receive cycles per minute. The mode is designed for rapid exchanges, but the "4" does not refer to the number of cycles.
   - **D: All these choices are correct**
     - This choice is incorrect because only the explanation related to four-tone continuous-phase frequency shift keying is correct. The other options do not accurately describe the "4" in FT4.

### Additional Information:

- **FT4** is a mode designed for fast exchanges, typically used in contesting and situations where quick communication is essential. It is part of the WSJT-X software suite, which includes other modes like FT8.
- **Frequency Shift Keying (FSK)** is a method of transmitting digital signals by changing the frequency of a carrier wave. In FT4, four different frequencies (tones) are used to represent data.