Answer:
Option A, The amount of Drug X given to the mice.
Step-by-step explanation:
In an experiment, an independent variable is not the subject of the experiment and hence the impact of independent variable is studied on the depended variable. An independent variable does not change or remains constant throughout the experiment.
Here in this case-
a) Independent variable - is the amount of drug (X) given to the mice having cancer.
b) Dependent variable - is the mice having cancer on which the effect of drug will be studied .
Therefore, option A is correct