In an experimental procedure an independent variable is the variable which can be manipulated or changed manually the effect of such changes can be observed on the dependent variable. The dependent variable is the outcome of the experiment.
According to the situation, carbon dioxide levels in the ocean water is the independent variable the effect of which can be observed on the number of plants that grow in the ocean. Number of plants that will grow in the ocean after the carbon dioxide levels increase or decrease will be the outcome of the study. Therefore, number of plants is the dependent variable.
Remember that a dependent variable is a variable that is being measured in a experiment so in this case your answer would be option C "plant height." Since how they are measuring plants height.
