If a Camera Lens Aperture Is Set To $f / 4$

If a camera lens aperture is set to $f / 4$ , then

A) the radius of the lens aperture is $1 / 4$ the focal length.
B) the radius of the lens aperture is 4 times its focal length.
C) the focal length of the lens is $4 \mathrm {~mm}$ .
D) the diameter of the lens aperture is 4 times its focal length.
E) the diameter of the lens aperture is $1 / 4$ the focal length.