Which of the following does a Snellen chart measure?

The Snellen chart is specifically designed to measure visual acuity, which refers to the sharpness or clarity of vision. It consists of letters or symbols printed in decreasing sizes, allowing the evaluator to assess how well a person can see at a distance. The results are typically expressed as a fraction, with the top number representing the distance from the chart (usually 20 feet) and the bottom number indicating the smallest size of letter that the person can read. For instance, a result of 20/20 is considered normal vision, meaning the individual can see at 20 feet what should normally be seen at that distance.

Other options provided do not pertain to vision assessment: blood pressure pertains to cardiovascular health, body mass index assesses weight relative to height, and respiratory rate tracks breathing patterns. Each of these metrics evaluates different aspects of health but does not involve the measurement of visual acuity, which is the sole function of a Snellen chart.