The correct answer is B. Tests experimental and control groups in parallel
Explanation:
In science, a controlled experiment is a type of experiment in which scientist intervene or manipulate variables. This often implies scientists use a control group in which the variable is constant and an experimental group that has a different treatment or changes in the variable studied. This allows scientists to evaluate the effect of the variable by comparing the results of both groups and thus confirm or discard a hypothesis. Therefore, a controlled experiment can be defined as on that "tests experimental and control groups in parallel".