"to determine if a certain nutrient improves vision among healthy adults, the experimental protocol to use is a(an)"

The experimental protocol which should be used is a controlled experiment. A controlled experiment is a scientific method used to minimise the bias and experimental errors. It involves a control and an experimental group of subjects. The experimental group of subjects includes the group of participants that will receive this nutrient tested. The control group is the group of participants that are under the exact same conditions as the experimental group ones but the only difference is that they do not receive the nutrient. That way, any difference observed in the vision of these two groups of subjects can be attributed to the nutrient.