To ensure that her experimental data are valid, the student should f use the same species of algae in all trials. This choice is critical because using the same species ensures that the results reflect the effects of pH changes on that specific type of algae, eliminating variability that could arise from differences among species.
While controlling final algae population sizes (g) and holding the acidity of the water constant (h) might also be important aspects of the experimental design, the primary factor is to use the same species to accurately assess how pH changes impact growth. Varying the temperature of the water (j) introduces another variable that could affect the results and should be controlled, rather than varied.
Therefore, the best answer for ensuring the validity of her experimental data is: f. use the same species of algae in all trials.