Timur and Juan are playing basketball. Timur shoots the ball and Juan blocks the shot. Timur says, "Wow, I didn't realize how tall you were until you did that!" Timur's comment refers to Juan's:

phenotype

Explanation:

The phenotype of an individual or an organism are the set of traits or characteristics that can be observed. It explains an organism's physical structure or morphology, behavior, developmental processes and other observable properties or characteristics.

Examples of phenotype include, height, hair color, eye color, skin color and other observable biochemical characteristics like hormone levels.

Timur commented on how tall Juan was, because the height is observable.