Final answer:
The correct answer for the experiment in question is option C, where the controlled variable is the type of solution used in the test tube, the independent variable is the known element in the solution, and the dependent variable is the color and wavelength values of the elements' emission line.
Step-by-step explanation:
Understanding the different types of variables in an experiment is crucial in experimental design. The independent variable is the one that is changed by the scientist. In the context of this experiment, the independent variable would be the known element in the solution, because this is what is being changed to observe the effect on the flame color and emission spectrum. The dependent variable is what is observed or measured; in this case, it is the color and wavelength values of the elements' emission line, which change as a result of the different elements used. The controlled variables are the factors that are kept constant to ensure that the results are due to the independent variable alone. Here, the type of solution used in the test tube is a controlled variable because it is kept the same throughout to prevent it from affecting the outcome.
Given these definitions, the correct answer for the variables in the given procedure is:
Controlled variable: The type of solution used in the test tube
Independent variable: The known element in the solution
Dependent variable: The color and wavelength values of the elements' emission line. Hence, the answer is option C.