E9A10:Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna?
Direct answer summary (2024-2028 NCVEC pool)
The official correct answer for E9A10 is Installing a ground radial system.
The key fact is Installing a ground radial system. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.