The correct answer is C: Multipath propagation cancels or reinforces signals. VHF signal strengths can vary greatly when an antenna is moved only a few feet because of multipath propagation - the signal arrives at your antenna via multiple paths (direct path plus reflections from buildings, terrain, etc.), and these paths can constructively or destructively interfere with each other. When multiple signal paths combine, they can add together (reinforce, making the signal stronger) or cancel each other out (making the signal weaker or disappear). Moving the antenna even a few feet can change the relative phase of these paths, dramatically changing the received signal strength. This is why VHF signals can be strong in one location and weak just a few feet away - you're experiencing different combinations of the multipath signals.