What is the primary classification of sex based on biological factors?

The primary classification of sex based on biological factors is best represented by anatomy. Biological sex is determined by physical characteristics such as reproductive organs, chromosomes (XX for females and XY for males), and other physiological factors that are present at birth. This classification encompasses aspects such as genitalia and internal reproductive structures, which are central to how biological sex has traditionally been understood in medical and scientific contexts.

Other options, while related to the broader context of sex and gender, do not strictly refer to biological classifications. Gender identity encompasses an individual's personal understanding and experience of their own gender, which may or may not align with their biological sex. Sexual orientation refers to whom an individual is attracted to, while physical features could include certain anatomical aspects but do not systematically classify biological sex in the same defined way as anatomy does. Thus, anatomy is the most precise and appropriate choice for categorizing biological sex.