Assertion (A): Sex of the children will be determined by what they inherit from their mother. Reason (R): Women have XX sex chromosomes.

According to the NCERT textbook, all children inherit an $X$ chromosome from their mother regardless of whether they are boys or girls. Therefore, the mother's contribution does not determine the sex of the child. Instead, the sex of the child is determined by what they inherit from their father: inheriting an $X$ chromosome from the father results in a girl ($XX$), and inheriting a $Y$ chromosome results in a boy ($XY$). Thus, Assertion (A) is false.

Reason (R) states that women have $XX$ sex chromosomes, which is true. In humans, women have a perfect pair of sex chromosomes, both called $X$.

Since Assertion (A) is false and Reason (R) is true, the correct option is D.

• Why A and B are incorrect: Both options require the Assertion to be true, which is not the case here.
• Why C is incorrect: This option states that the Assertion is true and the Reason is false, which is the exact opposite of the facts.