Which of the following statements is true?
a. the entropy of a system always decreases for a spontaneous process.
b. a spontaneous reaction is always a fast reaction.
c. perpetual motion machines are a possibility in the near future.
d. there is a "heat tax" for every energy transaction.
e. none of the above is true.

The correct answer is D.

Explanation:

In every process that includes an energy transaction or transition, inevitably a little amount of energy in heat form will be lost. Regardless if the process is spontaneous or not, the loss of energy (heat tax) will be present always. Also, it is impossible to make a perpetual machine, not always a spontaneous reaction will be fast, and only in certain cases the entropy could decrease during a spontaneous process.

Therefore the correct answer is D.