Answer:
D. to increase the reliability of his data
Step-by-step explanation:
For an experiment to be considerably reliable, it must be repeated and the same results must be visualized in all repetitions. This indicates that the experiment was performed correctly and that its results are reliable and replicable. If in one of the repetitions of the experiment a different result is visualized, it means that something is incorrect and the researcher should look for what is taking the reliability of his study.
For this reason, we can say that Darren repeated his experiment four times to increase the reliability of his data.