The sex of the baby is determined by the chromosomes contributed by the parents. Option 1, mother, is incorrect because the mother`s chromosomes do not determine the sex of the baby. Option 2, father, is the correct answer because the father`s sperm carries either an X or a Y chromosome, determining whether the baby will be male (XY) or female (XX). Option 3, both mother and father, is partially correct as the combination of the father`s sperm and the mother`s egg determines the sex of the baby. Option 4, grandmother, is incorrect as the sex of the baby is not influenced by the grandmother`s chromosomes. It is important to note that while the father`s contribution determines the sex of the baby, other factors in development, such as hormonal influences, also play a role in sexual differentiation.