Explanation: Nitrogen, or sometimes dry air, is placed in transmission lines primarily to prevent moisture from entering or condensing within the line (D). Moisture is highly detrimental to transmission line performance. Water is a lossy dielectric at radio frequencies, which increases attenuation and leads to power loss. More critically, condensed moisture can create conductive paths, lowering the dielectric strength of the line's insulation and leading to arcing, short circuits, and eventual failure. By maintaining a positive pressure of dry nitrogen, humid ambient air is effectively excluded. Regarding the other options: A) Skin effect is a property of the conductor itself at high frequencies and is not affected by the gas inside the line. B) While a dry gas like nitrogen does have higher dielectric strength than humid air, thus indirectly reducing the likelihood of arcing, its primary role in this context is to prevent moisture, which is the root cause of many arcing issues in lines. C) The standing wave ratio (SWR) of a line is determined by the impedance match between the source, line, and load. Nitrogen itself doesn't directly reduce SWR, though moisture can degrade the line's characteristic impedance, indirectly worsening SWR.