Final answer:
The doctor is referring to their son's sex, which is determined by reproductive anatomy.
Step-by-step explanation:
When the doctor tells Yuli and Ahmed that their baby is a boy, the doctor is referring to their son's sex. Sex refers to the biological or physical distinctions between males and females, determined by the presence of male or female genitalia at birth. While gender refers to the social and cultural roles and expectations associated with being male or female, sex is determined solely based on reproductive anatomy.