Final answer:
Einstein's equation E = mc² indicates that mass can be converted to energy, an idea originating from the theory of special relativity.
Step-by-step explanation:
Albert Einstein's equation E = mc² describes the fact that mass can be converted to energy and vice versa. This principle is derived from the theory of special relativity and embodies the idea that mass and energy are two forms of the same thing. Therefore, the correct answer to the student's question is C. Mass can be converted to energy.