Final answer:
A controlled variable is d) a variable that stays the same throughout an experiment. It is used to ensure that changes in the dependent variable are directly due to the independent variable.
Step-by-step explanation:
A controlled variable is a variable that stays the same throughout an experiment to ensure that any changes in the dependent variable are due to the manipulation of the independent variable and not some other factor.
In the context of scientific experiments, a controlled variable is d) a variable that stays the same throughout an experiment. This is in contrast to the independent variable, which is the one that the researcher changes or manipulates, and the dependent variable, which is the one that is observed and measured to see how it is affected by changes in the independent variable. For example, in an experiment to determine the effect of light on plant growth, the amount of water and type of soil used for all plants would be controlled variables, whereas the amount of light would be the independent variable, and the growth of the plants would be the dependent variable.