Answer:
A) the organism
Step-by-step explanation:
In an experiment, the independent variable is the variable that the experimenter changes or manipulates in the experiment in order to bring about a measurable response.
In this question involving the a student researching evolutionary relationships between different organisms who performed an ELISA test to analyze differences between cytochrome C proteins in different organisms. The DIFFERENT ORGANISMS used by the student is the independent variable because it is what the student changes in order to check out the difference in their cytochrome C proteins