Answer:
The correct answer will be-diet pill
Step-by-step explanation:
An independent variable is a parameter which can be modified or changed either on their own or manually. The independent variable directly influence the variable to be studied (dependent variable) in an experiment.
In the given question, since the variable which can be changed to control the height of the tomato plants is diet pills. Different concentrations of the diet pill must be supplied to the experimental samples and thus diet pill is the correct answer.