Final answer:
The client must stand 20 feet from the Snellen chart to perform a visual acuity test, as it is the standard testing distance to diagnose and evaluate vision impairments. The correct option is c) 20 feet.
Step-by-step explanation:
A client must stand 20 feet from the Snellen chart to test visual acuity. The Snellen chart is a tool used by optometrists for assessing a person's vision. It presents a series of letters of decreasing size, which a person with normal vision should be able to read from a distance of 20 feet.
The smallest line of text on a standard Snellen chart, which a person with normal vision should be able to read, subtends an angle of five minutes of arc at this distance. The chart helps to diagnose visual impairments, such as myopia or hyperopia, and is also used to determine how well eyeglasses or contact lenses correct someone's vision.