Professor Black is unsure whether the results of his experiment are valid, he will create a new operational definitions of his variables.
Answer: Option C
Step-by-step explanation:
In an experiment, if the results are unsure, to receive the results properly, there are only the variables which are independent can be changed and the dependent variables can not be changed.
So changing the definition or the meaning of the variables which are manipulative can have a direct affect on the experiment and can help achieve the results which are desired.