*Question G3C11:
Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?*  

*Correct Answer is  D: The D region*  

### AI Response: (may contain errors)  

The correct answer is D: The D region.

1. **Why the correct answer is correct:**
   - The D region of the ionosphere is the lowest layer and is present during daylight hours due to solar radiation. It is known for being highly absorbent of radio signals, especially those below 10 MHz. This absorption is primarily due to the increased ionization caused by solar ultraviolet and X-ray radiation, which enhances the density of free electrons that can absorb radio waves.

2. **Why the other choices are incorrect:**
   - **A: The F2 region**  
     - The F2 region is much higher in the ionosphere and is primarily responsible for long-distance HF radio wave propagation. It does not significantly absorb signals; instead, it refracts them back to Earth, allowing for long-range communication.
   - **B: The F1 region**  
     - The F1 region is also higher than the D region and is less absorbent. It primarily contributes to the refraction of radio waves rather than absorption. It merges with the F2 region at night.
   - **C: The E region**  
     - The E region is above the D region and can reflect radio waves, but it is not as absorbent as the D region. It plays a role in short-range HF propagation and sporadic E propagation but does not significantly absorb signals below 10 MHz.

3. **Useful rules of thumb:**
   - During daylight, the D region is the primary cause of absorption for frequencies below 10 MHz.
   - Absorption decreases with increasing frequency, so higher frequencies are less affected by the D region.
   - At night, the D region dissipates, reducing absorption and allowing lower frequencies to propagate more effectively.