For some reason (not yet understood) more male babies are conceived (born) than female. By childhood the number of boys and girls is the same. what is the possible explanation for the higher mortality (both before and after birth) of male babies?

Is there multiple choice options?

Maybe the X chromosome is an issue here, since males have only one X chromosome whereas females have 2. so, in males every mutation in chromosome X is seen in a phenotype while females are likely to be heterozygous and even if one of their X chromosomes is affected by mutation, the other would compensate.