VO2 max is the greatest rate at which the body can take in, transport, and use oxygen during intense, all-out exercise. It reflects the combined capacity of the lungs, heart, blood vessels, and muscles to deliver and utilize oxygen, and is often expressed as ml of O2 per minute (absolute) or ml per kilogram of body weight (relative). According to the Fick principle, VO2 max = maximum cardiac output × maximum arteriovenous oxygen difference, highlighting why it's a measure of overall aerobic capacity. The other statements don’t fit because VO2 max is about maximal oxygen use during strenuous activity, not average oxygen use during light activity, not heart rate, and not the small amount of oxygen dissolved in blood at rest.

Skinfold analysis relies on measuring subcutaneous fat at multiple locations rather than a single spot. Taking measurements from three or more sites captures how fat distribution varies across the body and allows the use of established prediction equations to estimate overall body fat more accurately. In practice, a three-site method is common for quick estimates, while seven sites are used for greater precision; measuring only one or two sites would not provide a reliable assessment.

Perceived exertion scales must match a person’s developmental level. The CERT, or Children’s Effort Rating Table, was developed specifically for children, using simple language and often visual cues that align with kids’ understanding of how hard a workout feels. It has been validated for pediatric use, so it’s appropriate for kids and not intended for adults. Other scales like the Borg 6–20 or the CR10 were designed with adults in mind and assume a certain level of numeracy and familiarity with the heart-rate relationship, which can make them less reliable or harder for children to use. The OMNI scale has child-friendly versions as well, but CERT is the one noted as suitable exclusively for children, making it the best answer for this item.

Newton's second law describes how motion responds to force: the net force acting on an object equals its mass times the acceleration it experiences. The acceleration points in the same direction as the net force, and its magnitude scales with both the net force and the object's mass—doubling the net force doubles the acceleration, while doubling the mass for the same force halves the acceleration. If no net force acts, the acceleration is zero, so the object moves with constant velocity or stays at rest. The equation also tells us the units: force is measured in newtons, which are kilograms meters per second squared. This relationship illustrates how mass, force, and motion are linked, and it’s distinct from energy conservation, action–reaction pairs, or equilibrium, which describes a balance of forces with zero net force.