I think the correct answer would be D. The tap water in the experiment is one the three test conditions of the independent variable, the type of water. The independent variable in a experiment is the one being manipulated or the one being changed. In this case, it is the type of water.
An experiment is performed to test the effect of three different types of water on the growth of a plant. The test is done by using tap water, distilled water, and carbonated water. Choose the statement below that describes the tap water in this experiment. a. The tap water is a controlled variable for all plants. b. The tap water is the control group for comparison. c. Tap water is the only test condition that will cause measurable changes in the dependent variable, plant growth. d. Tap water is one of the three test conditions of the independent variable, the type of water.