ANS:C - relieve stresses caused by thermal expansion.

In a heat exchanger, a floating head is provided primarily to relieve stresses caused by thermal expansion. Heat exchangers operate under varying temperatures, leading to thermal expansion and contraction of the components. The floating head design allows for some movement or displacement of the tube bundle or shell to accommodate these thermal stresses without imposing excessive mechanical loads on the equipment. While a floating head may indirectly contribute to other factors such as facilitating cleaning by allowing easier access to the internals of the exchanger or potentially increasing the heat transfer area by allowing for more effective cleaning and maintenance, its primary function is to accommodate thermal expansion and prevent mechanical damage to the exchanger. Therefore, the most relevant option is: "relieve stresses caused by thermal expansion."