(c) (A) is true, but (R) is false.
Assertion is correct — sex is genetically determined by chromosomes inherited from parents. But the Reason is false — a male child inherits his X chromosome from his mother, not his father; he inherits the Y chromosome from his father.
The key fact: men are XY, women are XX. Every child gets X from the mother. The father contributes either X (girl) or Y (boy). So the X in a son comes from the mother, making (R) false. Assertion (A) is true as the textbook clearly states sex is determined by genes (chromosomes) inherited from parents.