I think the answer will be C
Small, but normal variations in genes may produce proteins that work differently from those of other individuals ( friends or relatives). This may affect how we respond to different medicines. The extent to which these proteins functions varies from one individual to another. Example; certain painkillers only work when body proteins convert them from an inactive form to an active one. If these proteins work too fast, the drug will be eliminated from the body before it has time to work. To do their job the pain killers need to bind and modulate a target body protein, the receptor, such that if the target body protein is altered the pain killer might not be able to bind.
I would personally say stem cells as your answer.
