One of the X chromosomes in females is inactivated by

The correct option is "epigenetic markers".

X-chromosome inactivation (XCI) is a biological process in female mammals where one of the two copies of the X chromosome is packaged into transcriptionally inactive heterochromatin. This dosage compensation mechanism ensures that females, who have two X chromosomes, do not produce twice as many X-linked gene products as males, who have only one X chromosome.

This process of inactivation is initiated by a specific gene on the X chromosome called the X-inactive specific transcript (XIST) gene, which transcribes a long non-coding RNA. The XIST RNA coats the chromosome from which it is transcribed, recruiting various proteins that apply epigenetic markers.

These epigenetic markers include modifications such as:
1. DNA methylation: The addition of methyl groups directly to the DNA molecule (specifically at CpG islands), which helps silence gene expression.
2. Histone modifications: Alterations to the histone proteins around which DNA is wrapped (such as histone methylation and histone deacetylation), which compacts the chromatin structure into an inactive state known as a Barr body.

Because these changes alter gene activity without changing the underlying DNA sequence, they are classified as epigenetic. Therefore, epigenetic markers are responsible for maintaining the inactivation of the X chromosome.