Looking at the information given in the description, the experiment is testing for the effects of the different salts on the growth of the plants.
This means that the type of salt is the independent variable, or the variable being manipulated and tested.
To ensure fair results, some parts of the experiment would need to be kept the same, which is the control. The only answer choice that makes sense in this instance is A. water.
Answer: A. Explanation: They lose 1 electrons in their outermost shell. The proton to electron ratio become unbalanced where the proton overpowers. Therefore, leaving the ion positively charged.