Final answer:
Option (d), The gene responsible for X inactivation in female mammals is the XIST gene, which leads to the condensation of one of the X chromosomes into a Barr body.
Step-by-step explanation:
X inactivation is controlled by the XIST gene. This process occurs early in development when female mammalian embryos consist of just a few thousand cells. One X chromosome in each cell inactivates by condensing into a structure called a Barr body, which is visible in the nucleus.
The particular X chromosome that is inactivated is random, but once inactivation occurs, all cells descended from that cell will have the same inactive X chromosome. In human females and certain other mammals like the female fruit flies, this compensates for their double genetic dose of X chromosome genes.
Therefore, the correct answer to the student's question is: X inactivation is controlled by the XIST gene.