Sex chromosomes are:

A. the largest chromosomes on a karyotype

B. paired homologous chromosomes

C. present only after puberty

D. the 23rd pair of chromosomes

D. the 23rd pair of chromosomes

Explanation:

Humans have a total of 23 pairs of chromosomes. Out of a total of 23 pairs, 22 pairs are autosomes. Autosomes are the chromosomes that carry the genes for all the genetic traits but are not involved in the sex determination of the individuals.

The 23rd pair of chromosomes in humans is of sex chromosomes as these chromosomes carry the genes to regulate the gender of the individuals. A human female has two copies of the X chromosome as sex chromosomes while human males have one X and one Y chromosome as their sex chromosomes. The Y chromosome carries "SRY" gene that codes for testes determining factor and regulates the development of testes in the embryo.