A student writes in a laboratory notebook: I placed a piece of iron in a beaker of water and the iron sank to the bottom of the beaker. What the student wrote in the laboratory notebook is an example of a(n)

option d : from a warmer substance to a cooler substance

according to second law of thermodynamic, heat flows from higher to lower temperature region. heat is the transfer of energy from an object at higher temperature to an object at a lower temperature. so energy flows from warmer to colder objects.