The rate of turn can be increased and the radius of turn decreased by decreasing airspeed and shallowing the bank. Option C.
Turn radius depends on both airspeed and bank angle. The turning radius at any bank angle is directly proportional to the airspeed squared. Doubling the airspeed quadruples the turning radius, and trebling the airspeed quadruples the radius.
During a constant banked turn, increasing airspeed will decrease turn speed and increase turn radius. Airplanes fly fast, but the distance they fly is faster than their speed. As a result, it takes longer to complete a turn and slows down the turn.
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