Answer:
The controlled variables in an experiment are those variables which are kept same in the overall reaction in every trial. It is kept as a benchmark or standard for all the variables that testable samples are exposed with.
Timing of experiment: The timing of the experiment can be used as a controlled variable as a particular time constraint can be used to perform an experiment.
Duckweed population: The duckweed population should be kept as a constant to observe the impact of variables on the duckweed populations.
Environmental light: A particular environment light can be used as a standard under the influence of which the duckweed population shows growth.
Environmental temperature: A controlled standard temperature can be chosen for every trial to observe the growth of the duckweed.