Answer: Doubling the radius.
Explanation:
The volume of a cone can be found with the following formula:

Where "r" is the radius and "h" is the height of the cone.
Let's find the volume of the conical tent with a radius of 10.4 feet and a height of 8.4 feet.
Identifiying that:

You get this volume:

If you double the radius, the volume of the conical tent will be:

When you divide both volumes, you get:

Therefore, doubling the radius will quadruple the volume of the tent.