Answer:
The correct answer would be "they determine the gender or sex of an organism".
They are also termed as sex chromosomes as they are involved in determining the sex of the organism.
In humans, they form the 23rd pair of the chromosomes. Human females are homozygous for X chromosome that is, they are XX while human males are heterozygous for sex chromosome that is, they are XY.
Y chromosomes usually contain TDF (testes determining factor) or SDY (sex determining region Y) which determine the male gender or sex in humans.