Answer :
There is a probability that heat will flow from warmer to cooler substances,
and a probability that heat will flow from cooler to warmer substances.
The probability that heat will flow from a cooler to a warmer substance is
so small that it is ignored. It's accepted as a 'law' of thermodynamics that
heat always flows from higher temperature to lower temperature.
The correct answer to the question is : From hot body to cold body for no external work done.
EXPLANATION :
Before going to answer this question, first we have to understand the direction of heat flow.
The amount of heat flown between two bodies is dependent on the temperature difference between them. It is the temperature which determines the direction of heat flow .
Just like water flows from higher level to lower level , heat flows from a body at high temperature to a body at low temperature or from hotter part of a body to its colder part. Hence, the normal direction of heat transfer between two bodies is from hot body to cold body.
But heat may also flow from a colder part of a body to its hooter part if external work is done. For instance, we may take the heat flow in a refrigerator.
