Answer: sunlight.
Explanation: When trying to establish the level or extent of correlation or relationship which exists between two variables, the variables are classed as independent and dependent variables. The independent variable is referred to as the variable which causes a change in the value of the other variable (dependent variable). It is also known as the explanatory or predictor variable as it lead to changes in the dependent or predicted variable. In the scenario above, the independent variable is sunlight whose impact leads to changes in the growth level of the plant.