(D) Assertion (A) is false, but Reason (R) is true.
A is false because sex is determined by what children inherit from their father (X gives girl, Y gives boy), not the mother. R is true — women are XX.
The passage clearly states: "the sex of the children will be determined by what they inherit from their father." Since mothers are always XX, they can only pass an X chromosome — so the mother's contribution does not determine sex. The father's X or Y does. Reason (R) is a true fact but it actually explains why the Assertion is false (mother only contributes X, so she cannot determine sex).