The heat transfer will only occur in setup 2 and heat will only flow from C to D.
In a mixture between two substances of different temperatures, there is heat transfer, in which the one with a lower temperature receives this heat and the one with a higher temperature loses the heat.
In a system, the value of heat lost and received is equal.
So, heat will only flow in setup 2 since it is the only one that has two bodies with different temperatures, and it will flow from C to D because C has a higher temperature.
The representation of heat transfer can be done as follows:
C ⇒ D
Learn more about heat energy in: brainly.com/question/1495272