The correct answer is "radius."
The volume formulas for cylinders, cones, and spheres include the radius as a key component:
- Cylinder volume: \( V = \pi r^2 h \)
- Cone volume: \( V = \frac{1}{3} \pi r^2 h \)
- Sphere volume: \( V = \frac{4}{3} \pi r^3 \)
In these formulas, \( r \) represents the radius.