The answer is D. For an experiment, there is independent variable and dependent variable. Independent variable is the control variable that needs to change. Dependent variable is the one that need to be observed. So, generally speaking, in one experiment, one variable needs changing and others need to be constant.
In a controlled scientific experiment, A. only one variable can be kept constant. B. no variables are kept constant. C. all variables are kept constant. D. one or more variables are kept constant.