D. Gather more supporting evidence.
In an argumentative essay, the goal is to convince others to agree with your position. To make this happen, your claim must be backed up with valid supporting evidence. When doing research, it is important to understand the counterargument and what supporting evidence would be used. However, if there is more supporting evidence for the opposite side, you either need to change sides (which isn't an option) or gather more supporting evidence. It might also be useful to reevaluate your reasons for your opinion. Another reason may be better supported with evidence.