The correct option is D) Investigation.
By definition, there are two central variables present during an experiment,which is a scientific process in order to test or determine the impact of something.
One of them is the dependent variable, which means a variable that fluctuates according to other variables or factors. On the other hand, the independent variable is not controlled by other factors , it can stand on its own and is responsible for the changes caused in the aforementioned variable.
Finally, an investigation is a type of research that involves observation and collecting data but does not include control. This means that it involves examining evidence and check into a problem , statement,etc thoroughly but without depending on control or variables.