Answer:
d. subjects
Step-by-step explanation:
"Experimental research, often considered to be the “gold standard” in research designs, is one of the most rigorous of all research designs. In this design, one or more independent variables are manipulated by the researcher (as treatments), subjects are randomly assigned to different treatment levels (random assignment), and the results of the treatments on outcomes (dependent variables) are observed."
Reference: Pelz, Bill, and Herkimer County Community College. “Research Methods for the Social Sciences.” Lumen, 2019