The person should be a male.
Only male carries XY chromosomes, meanwhile female carries XX chromosomes. "Heterozygous for the X and Y chromosomes" means that the person have both X and Y chromosomes in their cells, therefore only males fits the description.
This is also why the birth rate of girl or a boy is always 50%, it is because the sex of a person is only determined by the sperm from a male. Each sperm either carries a X or a Y chromosome, never the both. Therefore, the chances of a X or a Y sperm fertilising an X egg is always 50%, also explains why both female or male must have at least one X chromosome because an egg always carry an X.