GMDSS operators should learn which of the following propagation "rules of thumb"?

Correct Answer: Option C — Longer distance = higher frequency, Shorter distance = lower frequency, Daytime = higher frequency, nighttime = lower frequency.

Effective HF communication relies on the ionosphere's state. Longer distances require higher frequencies to achieve greater skip distances. Daytime ionization necessitates higher frequencies to avoid D-layer absorption, while nighttime requires lower frequencies because the ionosphere's reflective layers change, making higher frequencies pass through into space instead of refracting.