How is the effective dosage of x-linked gene products balanced in placental mammals between females that carry two x chromosomes and males that carry only a single x chromosome?
> genes on the single male x chromosome are expressed at twice the rate of genes on the two female x chromosomes.
> only half of the genes on each x chromosome are expressed in female somatic cells.
> the paternally derived x chromosome is inactivated in all female somatic cells, so only the maternally derived x chromosome is expressed.
> one x chromosome is randomly inactivated in female somatic cells.

One X chromosome is randomly inactivated in female somatic cells.

Explanation:

Females in placental mammals such as humans have two copies of the X chromosome while the males have only a single X chromosome in their cells.

However, the cells of males and females of these organisms have the same effective dosage of X linked genes since one of the X chromosomes in somatic cells of these females is inactivated during early embryonic stages.

The inactivated X chromosome is present in the form of a compact object, called the Barr body in the cells of the females. Methylation of DNA is considered to be the possible mechanism responsible for the inactivation of the X chromosome.