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The action of a cycloplegic agent is accurately described as dilating the pupil and paralyzing the ciliary muscle of the eye. Cycloplegics are used primarily in ophthalmology to facilitate eye examinations and treatments. By dilating the pupil (a process called mydriasis), these agents allow for a better view of the interior structures of the eye, such as the retina and optic nerve, which is critical in diagnosing various conditions.

Paralysis of the ciliary muscle prevents accommodation, meaning the eye cannot focus on near objects during the examination. This dual action—pupil dilation and paralysis—helps the clinician perform a thorough assessment without the eye's natural reflexes interfering.

Other options do not accurately represent the effects of a cycloplegic agent, with constriction of the pupil being opposite to the intended action of these medications. The term "locks the pupil" is also misleading, as it implies a fixed state but does not encompass the effects of both dilation and paralysis. While the term "paralyzes the pupil" may suggest some aspect of the action, it fails to capture the significance of dilation in the therapeutic context. Therefore, the most complete and accurate description is that a cycloplegic agent dil