Answer:
The correct answer would be C. Algae that are placed in a yellow solution and kept in the dark.
In a scientific experiment, a control is a variable which is kept constant or unchanged throughout the experiment.
The results of other variables are compared with the control.
It increases the reliability of an experiment.
In the above experiment, algae must be kept in dark in order to prevent photosynthesis and thus, it prevents the utilization of carbon dioxide.
Similarly, the yellow color solution would also act as control as it would help in observing if any carbon dioxide is utilized from the solution. The utilization or removal of the same would convert the solution blue in color.
