Answer:
The correct answer will be- independent variable.
Explanation:
A scientific experimental design must include three types of variables which are independent variable (which can be manipulated), dependent variable ( which has to be tested and depending on the independent variable) and constant variable (which is kept constant).
In the given question, since the student wants to study the difference in the activeness of the goldfish in red and grey gravels, therefore, he performed the experiment with different gravels.
In this experiment, the independent variable is the color of the gravel as it can be changed as well as the dependent variable depends on this variable.
Thus, the independent variable is the correct answer.
