The correct answer is "A is false, but R is true."

The sex of the offspring in human beings is determined by the combination of sex chromosomes inherited from both parents. While it is true that women have two X chromosomes (XX), the major role in determining the sex of the offspring lies with the male.

Human males have one X chromosome and one Y chromosome (XY). The presence of the Y chromosome determines the development of male characteristics. If a sperm carrying an X chromosome fertilizes the egg, the resulting offspring will be female (XX). If a sperm carrying a Y chromosome fertilizes the egg, the resulting offspring will be male (XY).

Therefore, the presence or absence of the Y chromosome in the sperm determines the sex of the offspring. The female`s contribution is always an X chromosome, but it is the male`s contribution that determines whether the resulting offspring will be male or female.

Hence, while the reason (R) is true, the assertion (A) is false as females alone do not play a major role in determining the sex of the offspring.