Explanation: Trans-Atlantic radio signals, particularly in the High Frequency (HF) bands, rely on reflection from the Earth's ionosphere. The ionosphere's ability to reflect radio waves is directly tied to its ionization levels. **Sunspot activity (A)** is correct because sunspots are indicators of intense solar activity. During periods of high sunspot activity, the Sun emits more ultraviolet and X-ray radiation. This increased radiation ionizes the Earth's upper atmosphere (the ionosphere) more strongly, especially the F2 layer, which is crucial for long-distance HF propagation. A more densely ionized ionosphere reflects HF signals more efficiently, leading to stronger received signals over long paths like Trans-Atlantic links. Solar activity, including sunspots, follows an approximately 11-year cycle, directly correlating with the observed increase and decrease in signal strength. **Aurora borealis (B)**, while related to solar activity, typically causes absorption and distortion of radio signals rather than enhancing their strength for general long-distance communication. **Extratropical Advection (C)** is a meteorological term unrelated to radio propagation. **Perigean tide (D)** is an astronomical phenomenon concerning Earth's oceans and has no bearing on radio wave propagation.