The correct answer is D: Enough so that if the antenna falls, no part of it can come closer than 10 feet to the power wires. The minimum safe distance from a power line when installing an antenna is enough distance so that if the antenna falls, no part of it can come closer than 10 feet to the power wires. This accounts for the antenna's length and ensures safe clearance even if it falls. For amateur radio operators, this is a critical safety rule. The 10-foot minimum is a safety standard that prevents contact with power lines.