Which tool is essential for checking eye alignment?

The cover test is the essential tool for checking eye alignment because it assesses the coordination and alignment of the eyes. During this procedure, one eye is covered while observing the movement of the other eye. If the uncovered eye moves to fixate on a target when the cover is removed, it indicates that there may be a misalignment, such as strabismus. This test can help determine whether the eyes are properly aligned and working together, which is crucial for binocular vision and depth perception.

The other tools mentioned, while valuable for various aspects of eye examination, do not specifically assess eye alignment. An ophthalmoscope is primarily used for examining the interior of the eye, a pupil gauge measures pupil size, and a retinoscope assesses refractive error by observing the reflection of light off the retina. Therefore, the cover test specifically targets eye alignment issues, making it the correct choice for this question.