Answer:
C. amount of phosphate
Step-by-step explanation:
First, you need to know what a dependent variable and an independent variable are.
The independent variable is the one that influences the dependent variable to exhibit a response that influences, generating a result that can be observed by the researchers.
The dependent variable, on the other hand, is one that does not influence anything, but is influenced to generate a result, by the independent variable.
In the question above, the researchers want to test the hypothesis that the added phosphate will cause the marigolds to produce more flowers. According to this hypothesis, we can conclude that the researchers want to know the influence of the amounts of phosphate on the flowering of marigolds. In this case, we can say that the amount of phosphate is the independent variable of the experiment.