Answer: Option (a) is the correct answer.
Explanation:
An independent variable is defined as a variable whose value or which is controlled in a scientific experiment. Basically, an independent variable is able to change the value of dependent variable.
The value of an independent variable can be changed by the experimenter.
For example, nature of substances in a chemical reaction will act as an independent variable.
This is because if a substance is reactive in nature then in the presence of another substance cannot become unreactive. Hence, nature of a substance is an independent variable.
Thus, we can conclude that out of the given options the nature of the substances is the independent variable.
