Final answer:
The controlled variable in an investigation stays the same to provide a comparison.
Therefore, the correct answer is: option "Stays the same to provide a comparison".
Step-by-step explanation:
A controlled variable in an investigation is one that stays the same throughout to provide accurate comparison and ensure that observed changes are due to the independent variable's manipulation.
In an experiment, researchers manipulate the independent variable to study its effects on the dependent variable.
The controlled variables, on the other hand, are kept constant throughout the experiment to ensure that any changes observed are solely due to the manipulation of the independent variable.
For example, in a study testing the effect of a nutrient on algal growth in ponds, the amount of nutrient added would be the independent variable, the algal growth would be the dependent variable, and controlled factors might include temperature, amount of light, and volume of water, all of which need to stay the same for all test subjects.