Assertion (A): The sex of a child in human beings will be determined by the type of chromosome he/she inherits from the father.

Reason (R): A child who inherits 'X' chromosome from his father would be a girl (XX), while a child who inherits a 'Y' chromosome from the father would be a boy (XY).

Assertion (A) is true because in human beings, females possess a perfect pair of sex chromosomes ($XX$) and can only contribute an $X$ chromosome to the gamete. Males possess a mismatched pair ($XY$) and can contribute either an $X$ or a $Y$ chromosome. Therefore, the sex of the child is determined entirely by the chromosome inherited from the father. Reason (R) is also true because it correctly specifies that inheriting an $X$ chromosome from the father results in a female offspring ($XX$), while inheriting a $Y$ chromosome results in a male offspring ($XY$). Since (R) provides the precise biological mechanism that explains why the assertion is true, (R) is the correct explanation of (A). Distractors (b), (c), and (d) are incorrect because they fail to recognize this direct cause-and-effect relationship.