4 . B
When you rotate a right triangle about a leg, you get a cone:
One leg becomes the base radius, and the other becomes the height of
the cone. Call the long leg a and the short leg b and plug them into the
cone volume formula:
It’s also given that the area of the right triangle is 3, so
Plug ab = 6 into the expression for the volume of the cone and you can
solve for a: