Among the following, which method is used to decrease optic zone size?

Decreasing the diameter of a contact lens effectively reduces the optic zone size. When the diameter of a lens is reduced, it creates a smaller area of optical correction, effectively shrinking the optic zone. This alteration directly influences the lens's coverage over the cornea, allowing for adjustments in vision correction by narrowing the area through which light passes to the eye.

Changing the lens material, altering the anterior or posterior surface curvature, or modifying the sagittal depth may impact other aspects of lens performance, fit, and comfort, but they do not specifically target the size of the optic zone in the same manner as decreasing the diameter does. Thus, adjusting the diameter is the most straightforward and direct method to achieve a decrease in the optic zone size.