Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna?

A quarter-wave vertical antenna requires a ground plane to function as the 'other half' of the radiating system. When mounted directly on the earth, the soil's natural electrical resistance acts as a significant loss factor, turning transmitter power into waste heat rather than radiated RF. Installing a ground radial system provides a highly conductive path for the return currents. This drastically lowers the total ohmic resistance of the antenna system. As a result, more of the applied power is radiated into space instead of being lost to the dirt, thereby directly improving the antenna's overall efficiency. Shortening the element or isolating the coax shield would only degrade performance.