Concept:The sex of a baby is determined by which sex chromosome is carried by the sperm that fertilizes the egg.Explanation:The mother’s egg always carries an X chromosome.The father’s sperm may carry either an X or a Y chromosome.If a Y-carrying sperm fertilizes the egg, the baby is XY (boy).If an X-carrying sperm fertilizes the egg, the baby is XX (girl).Therefore, the probability of a baby being a boy or a girl depends on the father’s sex cells.Answer:A. father's sex cells