Final answer:
The test variable (IV) in the described experiment is the three different substances, as these are the variables being changed to observe their effect on the dependent variable, which is the melting point.
Step-by-step explanation:
In the described experiment, the test variable (independent variable (IV)) is B. The three substances. This is because the melting points of the different substances are being observed in a controlled environment, where the temperature, cone shape, and time in the cabinet are held constant. The independent variable is the factor that is changed by the scientist to observe how it affects the dependent variable—in this case, the melting points of the substances. As for melting points, these are significant because they represent the temperature at which a solid will turn into a liquid, which is a unique physical property for every substance.