Answer:
D). Use the same species in all trials.
Step-by-step explanation:
The student must 'use the same species of algae' in all the trials conducted during the experiment in order to ensure the validity of experimental data. A variation in the species may affect the authenticity of the results because every species of organism require the different environmental conditions and has varied nature and adaptability according to the environment. Therefore in order to produce authentic and reliable outcomes, the same species of algae should be used while observing how a variation in the pH level of water affects the growth rate of algae. Thus, option D is the correct answer.