ANS:C - turbulent

The maximum heat transfer rate is achieved in counter-current flow. In counter-current flow, the hot and cold fluids flow in opposite directions, which creates a higher temperature gradient along the length of the heat exchanger. This temperature gradient facilitates continuous and efficient heat transfer between the two fluids, resulting in a higher heat transfer rate compared to co-current flow. In co-current flow, the hot and cold fluids flow in the same direction, which can lead to a reduced temperature gradient along the length of the heat exchanger, thereby limiting the heat transfer rate. While turbulent flow generally enhances heat transfer compared to laminar flow due to increased mixing and turbulence, the specific flow arrangement (co-current or counter-current) still plays a significant role in determining the maximum heat transfer rate.