Buchholz relays are actuated by which of the following mediums?

Buchholz relays are protective devices used primarily in transformer applications to detect gas accumulation and oil movement within a transformer. They function by monitoring the oil that circulates within the transformer; when a fault occurs, such as an internal short circuit, gases are generated which accumulate in the relay.

The Buchholz relay operates based on the specific properties of the oil used in transformers. As gas accumulates in the oil, it causes an upward movement in the oil itself within the relay, triggering the relay mechanism. This makes Buchholz relays very effective in detecting issues early on and preventing more severe transformer damage.

In the context of the other options, water, air, and liquid sulfur do not serve the same function or have the appropriate properties for use in Buchholz relays. Transformers operate using specific insulating oils, which is why gas or oil is the correct medium for the operation of Buchholz relays.