What tool is specifically used to examine the retina of the eye?

The ophthalmoscope is the correct tool for examining the retina of the eye. It is designed to provide a magnified view of the interior structures of the eye, including the retina, optic disc, and blood vessels. This examination is crucial for detecting various eye conditions such as retinal detachment, diabetic retinopathy, and macular degeneration.

In contrast, the otoscope is used for examining the ear and is not suitable for viewing the retina. A sphygmomanometer is utilized for measuring blood pressure and has no application in eye examinations. The stethoscope is primarily used for listening to heart and lung sounds and does not have any connection to ocular assessments. Thus, the ophthalmoscope stands out as the appropriate instrument for retinal examination.