Explanation: For long transmission line runs, the primary concern is minimizing signal loss, or attenuation, which increases with cable length and frequency. Larger diameter coaxial cables inherently have lower attenuation per unit length than smaller diameter cables. This is because they feature thicker central conductors, which reduces resistive losses, and a greater separation between the conductor and shield (larger dielectric), which reduces dielectric losses. RG-8/U is a relatively large diameter (typically 0.405 inches) 50-ohm coaxial cable designed for high-power applications and longer runs where signal integrity is crucial. Its construction minimizes attenuation, making it the most appropriate choice among the options. Conversely, RG-58/U (approx. 0.195 inches) and RG-174/U (approx. 0.110 inches) are much smaller diameter 50-ohm cables. Their thinner conductors result in significantly higher attenuation, making them suitable only for very short runs or low-power applications. RG-59/U is also a smaller diameter cable (approx. 0.242 inches) and, importantly, is a 75-ohm cable, which is generally not used for standard amateur radio transceivers and antennas that operate at 50 ohms.