The statement "Heat cannot by itself flow from a body at lower temperature to a body at higher temperature" is a statement or consequence of the second law of thermodynamics. This law essentially states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. In simpler terms, it implies that heat energy cannot spontaneously transfer from a colder body to a hotter one.

Therefore, the correct option is :

Option A : second law of thermodynamics.