*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 to the question about what the "4" in FT4 refers to is:

**B: Four-tone continuous-phase frequency shift keying.**

### Explanation of the Correct Answer:
- **FT4** is a digital mode used in amateur radio that employs **four-tone continuous-phase frequency shift keying (CPFSK)**. 
- The "4" indicates that the mode uses four distinct tones to represent data, allowing for efficient transmission and robust performance in noisy conditions.

### Why the Other Choices are Incorrect:
- **A: Multiples of 4 bits of user information**
  - This choice is misleading. While FT4 does transmit data, the "4" specifically refers to the number of tones used, not the bits of information. The data is encoded in a way that does not directly correlate to multiples of 4 bits.

- **C: Four transmit/receive cycles per minute**
  - This is incorrect because FT4 does not define its operation in terms of cycles per minute. Instead, it focuses on the modulation scheme (four tones) rather than the timing of transmissions.

- **D: All these choices are correct**
  - Since choices A and C are incorrect, this option cannot be correct. The "4" specifically relates to the four-tone modulation scheme.

### Summary:
- FT4 is a digital mode characterized by four-tone CPFSK, which enhances data transmission efficiency and reliability. Understanding the modulation scheme is crucial for grasping how FT4 operates in amateur radio communications.