*Question G2C04:
What does the Q signal “QRL?” mean?*  

*Correct Answer is  D: “Are you busy?” or “Is this frequency in use?”*  

### AI Response: (may contain errors)  

The Q signal "QRL?" is used in amateur radio to inquire if a frequency is currently in use. 

1. **Correct Answer Explanation**:  
   - The correct answer is **D: “Are you busy?” or “Is this frequency in use?”**  
   - "QRL?" is a standard Q signal that operators use to check if a frequency is occupied. If you send "QRL?" to another operator, you are asking if they are currently using that frequency.

2. **Incorrect Choices Explanation**:  
   - **A: “Will you keep the frequency clear?”**  
     - This is not the meaning of "QRL?". Instead, it is more aligned with "QRM?" which asks if the frequency is clear.
   - **B: “Are you operating full break-in?” or “Can you operate full break-in?”**  
     - This refers to "QSK", which is the Q signal for full break-in operation, allowing operators to receive while transmitting.
   - **C: “Are you listening only for a specific station?”**  
     - This is not related to "QRL?". It does not correspond to any standard Q signal and is not a common inquiry in amateur radio.

3. **Useful Rules of Thumb**:  
   - Q signals are a shorthand way of communicating specific questions or statements in radio communication.  
   - Familiarity with Q signals can enhance communication efficiency and clarity among operators. 

In summary, "QRL?" is specifically used to ask if a frequency is in use, making it essential for maintaining proper operating etiquette on the air.