The universe's total amount of energy does not ever change, but the amount of energy that is available to do work varies tremendously. What statement best explains the changes in available energy?

Question

100% of energy is always converted with perfect efficiency
Energy transfer always occurs through heat
Energy constantly changes from one form to another
The only way to create energy is through nuclear reactions

Answer 1

The statement that best explains the changes in available energy is:

Energy constantly changes from one form to another.

This reflects the principle that while the total amount of energy in a closed system remains constant (as expressed by the law of conservation of energy), the availability of that energy to do work can change depending on the form it takes (such as kinetic, potential, thermal, etc.) and the processes it undergoes.