Which combining form indicates a female?

The choice of "Estr/o" as the combining form indicating a female is based on its association with estrogen, which is a primary female sex hormone. Estrogen plays a significant role in the development and regulation of the female reproductive system and secondary sexual characteristics. The prefix "estr" often relates to female reproductive health, making it the most directly relevant combining form in this context regarding gender.

The other options represent different medical terms that do not pertain specifically to the concept of "female." For instance, "Femor/o" relates to the femur or thigh bone, "Fibr/o" refers to fibrous tissue, and "Erg/o" pertains to work or activity in a broader sense. These terms do not carry the gender-specific connotation that "Estr/o" does, which is rooted in the biological functions and characteristics associated with females.