The correct answer is D. SF6.
SF6 (sulfur hexafluoride) is a gas that is used as an insulating medium in high-voltage equipment. It is non-flammable, non-toxic, and has a very high dielectric strength. This makes it ideal for use in circuit breakers, which need to be able to interrupt high currents without arcing.
Water is a good conductor of electricity, so it is not suitable for use in circuit breakers. Oil is also a conductor of electricity, but it is less conductive than water. This makes it a better choice for use in circuit breakers, but it is still not ideal. Air is a good insulator, but it is not as good as SF6. This means that air circuit breakers are not as reliable as SF6 circuit breakers.
In conclusion, the correct answer is D. SF6.