Natural processes are
A. Reversible
B. Irreversible
C. Isothermal
D. Adiabatic

Hint: Here the concept of spontaneity will be used. This is derived from the second law of thermodynamics. This law gives an idea about the entropy change. If the entropy increases, the change is spontaneous. Any spontaneous change is an irreversible change. For this question, we shall first understand what are the changes accompanied in a natural process and then we shall compare them with the options given to get the answer.

Complete answer:
The second law of thermodynamics states that it is a natural tendency of a system to progress in a more disordered state. In other words, if the entropy of a system increases, it favors the natural tendency and the change is spontaneous. Any spontaneous change is an irreversible change. This is because a system would not prefer to progress in an ordered state. Any spontaneous change is an irreversible change.
Natural processes can be both isothermal and adiabatic. The phase changes are isothermal processes while the vertical descent of air on cooling is an adiabatic process. Natural processes are spontaneous. There is no human interference and the processes occur by themselves. However, if we want the change to proceed in the opposite direction, we have to use external forces and equipment. Thus, the natural processes are irreversible.
For example, water from a tank located at a height flows through the pipes to the containers on ground by itself. There is no human aid and the process is spontaneous. As soon as a difference in the water potential is registered, the water starts moving. However, it will not go back up to tank through the same pipe without any external influence.
Hence, it is irreversible.

Note:
Irreversibility does not guarantee a chemical change. All chemical changes are irreversible. We cannot get back the original reactants from the products without any external influence. However, irreversibility is more linked to the entropy change. The entropy gives a measure of disorderliness in a system. If it increases, then the change is irreversible.