Which eye drop inactivates the sphincter muscle of the iris?

The correct answer is the cycloplegic eye drop. Cycloplegics are medications used to temporarily paralyze the ciliary muscle of the eye, which is responsible for focusing. By causing relaxation of this muscle, cycloplegics also indirectly affect the sphincter muscle of the iris, which contributes to pupil dilation. As a result, the muscles that constrict the pupil are inhibited, leading to wider dilation.

In contrast, other types of eye drops serve different purposes. Anesthetic eye drops are used to block sensation in the eye but do not have an effect on the iris muscles. Miotic drops cause constriction of the pupil by stimulating the sphincter muscle, which is the opposite of what a cycloplegic does. Mydriatic drops primarily work to dilate the pupil, usually by relaxing the sphincter muscle, but they don't specifically paralyze it the way cycloplegics do.

Therefore, cycloplegics are specifically designed to inactivate the sphincter muscle of the iris as part of their mechanism of action, making them the correct choice for this question.