Step 1: Picture the mirror as a slice of a sphere.
A spherical mirror is a curved piece cut out from a larger hollow sphere, and its edge traces out a circle.
Step 2: Identify what aperture measures on that circle.
Aperture is simply the straight line distance across that circular boundary, in other words its diameter, describing how big or small the usable reflecting surface is.
Step 3: Distinguish it from the other listed terms.
It is not the curvature of the surface, not the focal length, and not the object distance, those are separate quantities entirely, aperture only concerns the physical size of the mirror opening.
\[ \boxed{\text{Diameter of the reflecting surface}} \]