In the western US, where do rain shadows most frequently occur?

Rain shadows occur on the leeward, east side of mountain ranges. This phenomenon happens when moist air rises over the mountains on the windward side, cools, and loses its moisture as precipitation. Once the air passes over the peak and descends on the leeward side, it becomes drier and warmer, creating conditions conducive to a rain shadow effect.

Thus, the correct answer is found in the understanding of how mountain ranges influence precipitation patterns in a region. In the western U.S., the Sierra Nevada and the Cascades exemplify this, where areas on the eastern sides of these ranges often experience significantly less rainfall than the western sides that are exposed to moist air coming from the Pacific Ocean.