Explanation: At microwave frequencies, conventional coaxial transmission lines experience significant signal loss due to two primary factors: the skin effect, which forces current to the outer surface of conductors, increasing effective resistance; and dielectric losses within the insulating material between the center and outer conductor. These losses become substantial as frequency increases. Waveguides, conversely, are hollow metal tubes that primarily use air as their dielectric, which has extremely low loss. While skin effect still occurs on the waveguide walls, the absence of a solid dielectric core and the different, highly efficient propagation mode result in considerably less attenuation. This makes waveguides the superior choice for transmitting microwave energy over distances with minimal signal degradation. A) They are generally *larger* than coaxial lines at a given frequency, as their dimensions are proportional to the wavelength. C) Due to their larger size and rigid construction, waveguides can often be *heavier* than equivalent coaxial lines. D) Moisture is indeed a problem; it can cause significant signal loss and reflections. Waveguides are often pressurized with dry air or nitrogen to prevent moisture ingress.