All Female Mammals Have One Active X Chromosome Per Cell

Multiple Choice

All female mammals have one active X chromosome per cell instead of two. What causes this to happen?

A) Activation of the XIST gene on the X chromosome that will become the Barr body.
B) Activation of the BARR gene on one X chromosome, which then becomes inactive.
C) Inactivation of the XIST gene on the X chromosome derived from the male parent.
D) Attachment of methyl (-CH₃) groups to the X chromosome that will remain active.