Final answer:
The item identified as a controlled variable is C) Procedures. Controlled variables are kept constant to isolate the effect of the independent variable on the outcome in an experiment.
Step-by-step explanation:
The items from the list provided that are controlled variables are C) Procedures. Controlled variables refer to the factors in an experiment that are kept constant to ensure that any changes in the outcome can be attributed to the manipulation of the independent variable only. In a hypothetical experiment where a psychologist studies the effect of tableware size on food consumption, the procedures would include the method of serving the meal, the environment in which the meal is eaten, and how the amount of food consumed is measured. These procedures should be kept the same for both groups to ensure that any differences in food consumption can be linked to the tableware size - the experimental variable - rather than other factors.
Other elements of an experiment, such as the population, sample, experimental units, explanatory variable, response variable, and treatments can vary or be manipulated according to the research question. But controlled variables must remain unchanged to maintain the integrity of the experiment.