Which part of the central nervous system is used to serve as the main communication link between the body in the brain

The main communication link between the body and the brain would most likely be the spine. The reason for this is that the signals which are produced and received in the brain, get first distributed over a number of areas in the brain and then sent down through the thalamus and down the medulla oblongata and the other nuclei that are present there. These go then through the spine towards our limbs and internal organs.