Final answer:
The law of conservation of energy states that energy cannot be created or destroyed, but it can be transformed in a closed system. In any energy conversion, kinetic energy is always present, which aligns with option B as the correct answer.
Step-by-step explanation:
The question asks about the principle that energy cannot be created or destroyed in a certain condition, but it can be changed in form. This principle is known as the law of conservation of energy, which states: during a chemical or physical change, energy can be neither created nor destroyed, though it can be changed in form. According to this law, in an energy conversion, some form of energy is always present. The correct answer to fill in the blanks would be: Energy cannot be created or destroyed in a closed system, but it can be transformed. In an energy conversion, kinetic energy is always present. Hence, the correct option is B. Closed system; transformed; kinetic.