Which tool is essential for completing an eye exam?

The ophthalmoscope is the essential tool for completing an eye exam because it is specifically designed to allow the examination of the interior structures of the eye, including the retina and the optic nerve. This instrument uses a light source and a series of lenses to provide a clear view of the eye's components, making it indispensable for diagnosing various eye conditions, assessing ocular health, and detecting systemic diseases that may manifest in the eyes.

In contrast, while a thermometer is useful for measuring body temperature, an otoscope is intended for examining the ear canal and tympanic membrane, and a stethoscope is primarily used for listening to heart and lung sounds. These tools serve important functions in veterinary medicine, but they do not provide the necessary capabilities for a comprehensive assessment of the eye. Thus, the ophthalmoscope stands out as the correct answer for this specific examination purpose.