Correct answer: B — Length of line
What determines corona inception
Corona starts when the electric field at a conductor surface becomes strong enough to ionize the surrounding air. The critical corona voltage is therefore a field-threshold quantity. It depends on conductor geometry, spacing, surface condition and air conditions.
Bundling divides a phase conductor into several sub-conductors. The number of sub-conductors and their spacing change the distribution of charge and the maximum electric field at their surfaces. The distance between phase groups also affects the surrounding electric field. These geometric changes can therefore alter the corona-inception voltage.
Why line length is the answer
Extending an otherwise identical, uniformly energized conductor increases how much conductor is present. It does not, by itself, change the local cross-sectional geometry or the critical surface field. Consequently, total line length is not an independent local corona-inception parameter in the standard model used by this question.
Length can affect total corona power loss. If comparable corona loss occurs per kilometre, adding more kilometres increases the total loss. That is a different quantity from the voltage at which corona starts. Real lines can have nonuniform weather, voltage profiles and surface defects; the stated comparison assumes those local conditions are held constant.
The key distinction is therefore inception voltage versus accumulated loss. A simple single-conductor formula should not be treated as an exact bundled-conductor formula merely by replacing its radius with an equivalent bundle radius.
