Sound is eternal because it is produced refers to which of the following Hetvabhasa?

The reason proves the very opposite of what is to be established, which is Viruddha (the contradictory reason).

The thesis claims sound is eternal, while the mark offered is that it is produced.

Whatever is produced is invariably non-eternal, so the mark pervades non-eternality.

Thus the reason actually establishes non-eternality, against the intended eternality.

A hetu that proves the contradictory of the sadhya is the Viruddha hetvabhasa.