The correct answer is c. The results about the benefits of using a vitamin E cream for knee pain are questionable because the researcher didn't have a control group.
Step-by-step explanation:
In medical research and especially when products, treatments or similar are tested it is necessary to have one control and one experimental group. In this way, the researcher uses the product with one of the groups (experimental group) and compares the result of this group with the one obtained in a control group that does not receive the treatment or product tested. According to this, the medical researcher that tested a cream with vitamin E would not be valid because the researcher did not have a control group and therefore the benefits he/she determine are questionable as these were not compared to the ones in the control group.