The answer is the D. "Observation" is not a variable in an experiment.
Explanation: Research variables are factors that can be manipulated, measured or those that we try to explain through other variables. "Observation" is not a variable in an experiment, but it is the first step and fundamental part in the scientific method, in which the experiments are contained (experimentation stage). Observation is the information recorded during an experiment, obtained through the senses or through the registration of information on scientific instruments.