Answer:
The correct option is option C, nucleus
Explanation:
As the scenario described in the question, the medicine X functions on the DNA as it's characteristic shows that it is damaging the DNA. The DNA is present inside the nucleus of a cell. Hence, for a medicine to function on the DNA, the medicine must enter the nucleus of the cell as it is the location where the DNA is present.
Other options like, option D, is not correct because the process of conversion of mRNA into proteins takes place in the ribosomes and DNA is not present there.
