The correct answer is A: Installing a ground radial system. Installing a ground radial system improves the efficiency of a ground-mounted quarter-wave vertical antenna by reducing ground losses. Radials provide a low-resistance path for return current, reducing power lost in the ground. A quarter-wave vertical needs a ground plane (radials) to work efficiently. Without radials, ground losses are high because the return current must flow through lossy soil. Radials create a low-resistance ground plane, reducing losses and improving efficiency. More radials and longer radials generally improve efficiency. A good radial system can significantly improve vertical antenna efficiency, especially on lower frequencies where ground losses are more significant.