Which of the following represents a phenotype?

A phenotype refers to the observable physical or biochemical characteristics of an organism, which are determined by its genotype and the environment. In this case, "Brown fur color in mice" represents a specific trait that can be seen and measured in the mice. It describes the external appearance and can be influenced by genetic factors.

The other choices refer to genotypes, which represent the genetic constitution of an organism. For instance, AA, Aa, and aa denote different combinations of alleles, but do not provide information about the observable traits of the organism. Therefore, the selection of "Brown fur color in mice" correctly identifies a phenotype, as it describes a visible characteristic of the mice in question.