Dosage compensation is an epigenetic mechanism by which gene expression is equalized in both male and females where unequal copies of sex chromosomes are present. In humans, the females contain two X chromosomes (XX), while the male contains only one X chromosome. So, in order to balance the gene expression in females and to equalize it with males, genes present in one of the X chromosomes are silenced.
Hence, the correct option is (d).
Dosage compensation is about the presence of one extra chromosome and to repress it to equalize the gene expression in both males and females.
Hence, the option (a) is incorrect.
Inactivated X chromosome can be a part of heterochromatin, but dosage compensation itself does not silence the genes present in heterochromatin.
Hence, the option (b) is incorrect.
Enhancers have no role in dosage compensation.
Hence, the option (c) is incorrect.