Which of the following substances is used as the gaseous dielectric in high voltage transformers?

In high voltage applications, such as transformers, circuit breakers (specifically sulphur hexafluoride circuit breakers), switchgear (specifically high voltage switchgear), radar waveguides, etc., dielectric gases are employed as electrical insulators.
In high voltage transformers, ${\mathrm{SF}}_6$ is employed as a gaseous dielectric because of its inertness.