Q.Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?

Concept: Beyond-the-horizon (long distance) communication using sky waves relies on reflection of the radio wave from the ionosphere back to Earth. This reflection is efficient only for a limited band of frequencies — roughly the High Frequency (HF) band, 3 MHz to 30 MHz.

Checking the options:

• (a) $10$ kHz — far too low; such very-low-frequency waves are not efficiently radiated by antennas of practical size and are instead associated with ground-wave propagation, not ionospheric skip.
• (b) $10$ MHz — lies within the HF band, so it is reflected by the ionosphere and can travel beyond the horizon via sky waves.
• (c) $1$ GHz $=1000$ MHz — far above the HF band; such high-frequency waves penetrate the ionosphere rather than reflecting off it (this is why satellite/space communication uses these bands).
• (d) $1000$ GHz — even higher frequency; penetrates the ionosphere just like (c), and is additionally absorbed strongly by the atmosphere.

Only $10$ MHz falls in the ionospheric-reflection window.

Sky-wave (beyond-horizon) propagation needs a frequency the ionosphere can reflect — the HF band, roughly 3–30 MHz. Very low frequencies aren't practically radiated for this mode, and frequencies well above 30 MHz pass through the ionosphere instead of bouncing back.

• Assuming higher frequency always means better/longer-range communication — very high frequencies actually escape through the ionosphere instead of reflecting.
• Confusing sky-wave (ionospheric) propagation with ground-wave or space-wave (line-of-sight) propagation, which apply to different frequency ranges.
• Picking the lowest frequency (10 kHz) assuming low frequency = long range, without checking that it must also actually reflect off the ionosphere.