Final answer:
The second law of thermodynamics declares that the total entropy of an isolated system will not decrease but will either stay the same or increase, leading to spontaneous heat transfer from hotter to colder bodies.
Step-by-step explanation:
The second law of thermodynamics states that the total entropy of an isolated system never decreases; it either increases or remains constant. This principle implies that heat will spontaneously transfer from an object with a higher temperature to an object with a lower temperature, which is why the correct statement is: Entropy of an isolated system never decreases.
Another way to express this law is by stating that the total entropy of a system and its surroundings always increases in a natural process. Since entropy measures the level of disorder, this law also suggests that processes involving heat transfer lead to an overall increase in the universe's disorder.