The correct answer is C: Soil conductivity. Soil conductivity determines ground losses for a ground-mounted vertical antenna operating on HF. Higher soil conductivity means lower resistance, which reduces ground losses and improves efficiency. Ground losses occur because return current flows through the soil. Soil with high conductivity (like saltwater-saturated soil or moist clay) has low resistance, so losses are low. Soil with low conductivity (like dry sand or rock) has high resistance, so losses are high. This is why vertical antennas work much better near saltwater (high conductivity) than in dry desert areas (low conductivity). Soil conductivity is the primary factor determining how much power is lost in the ground versus radiated.