Answer: D. Plant growth
Explanation:
An independent variable is the one which can be changed or manipulated in a given experiment. The effect of such change can be observed on the dependent variable. A dependent variable is the one which cannot be changed or manipulated manually. The changes occurring in the dependent variable can be observed to deduce the outcome of the experiment.
According to the given situation, a new fertilizer is the example of the independent variable the effect of which can be observed on the plant growth which is a dependent variable.