Which of the following is an example of a biometric?

Heart rate variability is considered a biometric because it quantifies a physiological characteristic related to the autonomic nervous system and heart function. Biometric measurements are typically related to physiological or behavioral characteristics that can be measured and analyzed, often to assess health or fitness levels.

Heart rate variability specifically provides insights into how well the body is responding to stress, recovery, and overall cardiovascular health. It reflects the time variation between heartbeats and serves as a non-invasive method for evaluating the body's regulatory systems in response to various stimuli.

The other options listed, while useful for assessing different aspects of fitness and health, do not classify as biometrics in the same way. Body composition analysis provides information about the proportions of fat and lean mass but does not measure a specific physiological process. Flexibility assessment focuses on the range of motion in joints and muscles, and muscle strength tests evaluate the maximum force exerted by a muscle group. Each of these assessments provides valuable data but centers around physical performance or measurement rather than on the physiological characteristics that biometrics target.