Answer:
at least three separate plants
Explanation:
Testing on 3 separate plants would show that the experiment is repeateable. It shows that the independent variable or what they want to test can be done several times.
It is important to note though that the plants need to be of the same species and at least at the same development stage. This is because you would want to ensure that only the independent variable is what caused any marked change in the plant.
In addition, all three plants should be treated the same with regards to other variables like type of soil, amount of water, even the amount of sunlight it receives as this would also be an intervening variable if not controlled. (thus, they are called controlled variables.)
