Answer:
36-45 inches
Explanation:
The regulation for all forms of these types of objects with dangerous voltage and the potential to discharge is generally 36-45 inches, in which a perimeter is to be formed with appropriate precautions and markings with NON conductive materials. . .you know, so electricity doesn't discharge, take the nearest conductor, rebound & amplify, and strike the nearest guy dead.
You have one answer that matches this regulation. . .3 feet.
However, I wouldn't rule out 5 feet.
The other options are ludicrous and will increase the danger.
Your first and best option is 3 ft, but if you need a last-resort, 5ft is also viable.