What aspect defines the phenotype of an organism?

The phenotype of an organism is defined as the observable characteristics or traits that result from the interaction of its genetic makeup with the environment. This includes physical appearances such as color, size, and shape, as well as behavioral traits. While the genetic makeup of an organism contributes to its potential traits, the phenotype specifically reflects those traits that can be observed at any given time, which makes it distinct from the underlying genetic information.

In contrast, the other options refer to different concepts within genetics and biology. The genetic makeup (genotype) influences the phenotype but does not define it directly. The number of chromosomes is a measure of genetic material but does not encompass the characteristics an organism displays. An organism's ability to adapt relates more to its evolutionary fitness and survival rather than the immediate observable traits that comprise its phenotype. Thus, observable characteristics are the key defining aspect of phenotype.