**Core Concept:** The sex chromosomes (X and Y) determine the sex of an individual in humans. The morphology and function of these chromosomes play a crucial role in sexual development and differentiation.

**Why the Correct Answer is Right:** Spermatogenesis is the process by which spermatozoa are produced in the testis. During this process, spermatogonia, a type of male germ cell precursor, undergo meiosis (a type of cell division) to form haploid cells called spermatids. These spermatids further differentiate into mature spermatozoa. The sex of a child is determined by the number and type of sex chromosomes the germ cells carry. In humans, male germ cells carry either one X chromosome and one Y chromosome (XY) or two Y chromosomes (YY). Female germ cells carry two X chromosomes (XX). The correct answer highlights the role of sex chromosomes in determining the sex of offspring.

**Why Each Wrong Option is Incorrect:**

A. This answer incorrectly refers to the presence of "Y" chromosomes, which is not the main point of the question. The question is about the basis of sperm bearing X or Y chromosomes, not just the presence of Y chromosomes.
B. This answer incorrectly states that the basis is "sex chromosomes," but the main focus is on the presence of either one X and one Y (XY) or two Y chromosomes (YY) in sperm.
C. This answer incorrectly implies that the basis is "sex chromosomes," but the question is about the presence of either one X and one Y (XY) or two Y chromosomes (YY) in sperm.
D. This answer incorrectly refers to "spermatozoa," which is not the main point of the question. The question is about the basis of sperm bearing X or Y chromosomes.

**Clinical Pearl:** The presence of XY chromosomes in sperm is crucial for male sexual development and fertility. The Y chromosome contains several genes that play a key role in male development, including SRY (Sex-determining Region Y), which initiates testes formation and testosterone production. The presence of X chromosomes indicates female sex development and infertility in males. Understanding these concepts is essential for understanding human sex determination and male fertility.