Final answer:
The equation t = 2m + 40 represents a direct relationship between the substance's temperature and the time it was heated, as the temperature increases with time.
Step-by-step explanation:
The equation t = 2m + 40 represents the relationship between the temperature, t, of a substance and the time it was heated, m. In this equation, a change in time, m, will result in a direct change in temperature, t. Specifically, as the time the substance is heated, m, increases, the temperature, t, will also increase. Therefore, the correct answer is c) Temperature increases with time.