The correct answer is C: It requires special techniques to prevent moisture in the cable. A disadvantage of air core coaxial cable (compared to foam or solid dielectric) is that it requires special techniques to prevent moisture in the cable. Air core cable uses air as the dielectric (with spacers to keep the center conductor centered), which means water can easily enter and flow through the cable if not properly sealed. Foam and solid dielectric cables are less susceptible to moisture because the dielectric material helps block water entry. For amateur radio operators, air core cable offers lower loss but requires careful installation and sealing to prevent moisture problems.