The correct answer is B: Foam dielectric coaxial cable has lower loss per unit of length. A significant difference between foam dielectric and solid dielectric coaxial cable (assuming all other parameters are the same) is that foam dielectric has lower loss per unit of length. The air in the foam reduces dielectric losses. Foam dielectric coax has air bubbles in the dielectric material, which reduces the effective dielectric constant and dielectric losses. Solid dielectric has more material, so more dielectric loss. Foam dielectric typically has 20-30% lower loss than equivalent solid dielectric coax. This lower loss is why foam dielectric coax is popular for long feed lines or high-power applications. The trade-off is that foam dielectric might have slightly lower maximum voltage rating, but the lower loss is usually more important.