Who contributes to determining the sex of a baby?

The correct choice indicates that the father's sperm is responsible for determining the sex of a baby, and this is based on genetic principles. Human sex determination relies on the combination of sex chromosomes contributed by both parents.

Each human egg produced by the mother contains one X chromosome. The father contributes sperm that can carry either an X or a Y chromosome. If the sperm carrying an X chromosome fertilizes the egg, the resulting baby will be female (XX). Conversely, if the sperm carrying a Y chromosome fertilizes the egg, the baby will be male (XY). Therefore, it is the specific type of sperm from the father that ultimately decides the biological sex of the baby.

Other options, while relevant in various contexts, do not directly influence the genetic determination of sex. The mother's egg contributes only an X chromosome, the womb's environment plays a role in fetal development but not in sex determination, and the timing of conception does not alter the genetic makeup that determines sex.