In summer, the shore is warmer than the river. Hence air above the shore is at high temperature compared to the temperature of the air above of the river. The warmer air particles flow from river to the shore due to convection. Therefore, there is a mild wind at the shore of a calm river. Whereas during winters it’s opposite. The temperature of the air above the river is warmer compared to that of air above land. Thus, convection current flows from land to river.
Thus, option (b) is the correct option.