Answer:
The response variable is; reduction in the pain associated with sickle-cell disease.
Step-by-step explanation:
Note, a response variable (dependent variable) simply refers to resulting effect (either significant or insignificant effect) of the independent variable on the subjects of the experiment.
In other words, the effect of the drug hydroxyurea after the experiment would show whether it leads to or not lead to a reduction in the pain associated with sickle-cell disease.