Answer:
Consumption of moderate doses of St. John's Wort
Explanation:
A variable that changes without depending on the other variable is called the independent variable and the variable that changes relative to the independent variable is called the dependent variable.
Here the effect of St. John's Wort on memory is being studied.
So, the independent variable would be the consumption of moderate doses of St. John's Wort and the dependent variable would be the effect it has on the person's memory.