Which of the following best describes physiological variation in biology?

Variation means the differences between individuals of the same species. Physiological variation deals with how the body WORKS on the inside — its processes and functions — rather than how it looks.

Common examples in humans are blood group (A, B, AB, O), the ability or inability to roll the tongue, tasting or not tasting PTC, sickle-cell status, and differences in how fast people digest food or fight off disease. You cannot see these by simply looking at a person; they show up in what the body does.

The other options describe other kinds of variation. Differences in physical appearance, such as height, skin colour or hair type, are morphological variation. Differences in the genes themselves are genetic variation, which is the underlying cause rather than the description asked for. Differences in how individuals act or relate to others are behavioural variation.