Answer: The scientist tests two plants with the new plant food, and two plants he grows without plant food.
Explanation:
A control is a sample which is kept constant in an experiment. It is kept as a benchmark for comparison with the test variables to observe the changes in the test variables. The use of control in an experiment is valid as it helps in determining the effect of one variable over the other or if the sample is not treated with any variable.
The scientist tests two plants with the new plant food, and two plants he grows without plant food. is the correct option. It is valid as in this two control samples are kept like two plants which grows without plant food used to compare with those who receive the plant food. Thus it will highlight the effect of plant food on growth.