Answer:
the drug itself
Explanation:
The independent variable is the variable that changes irrespective of other variables.
When a change in variable is caused due to the change in the independent variable is called the dependent variable.
Here, whether a drug is effective in relieving the symptoms of arthritis is being determined. The drug will be considered effective only if the treatment group show the desired effects. The placebo control group should not show any effects of the drug. Hence, the drug is the independent variable.