A ship has been communicating effectively on 16 MHz during daylight hours with a shore station at a distance of 3500 miles. Toward late afternoon and evening what effect would be noticed?

Correct Answer: Option C — Communications should gradually deteriorate and become impossible on this frequency at night.

As daylight fades, the ionosphere's D-layer dissipates and the F-layer height changes, causing the Maximum Usable Frequency (MUF) to drop. A 16 MHz signal, which is effective for long-range daytime propagation, will likely exceed the MUF at night, causing the signal to pass through the ionosphere rather than reflecting.