Woodruff key:

• A key made from a cylindrical disc having segmental (Semi-circular) cross section is known as woodruff key.
• Woodruff keys require some means for this such as retaining rings, clamp, collar etc.
• It requires deep penetration into shaft because of semi-circular shape and weakens the shaft and hence employed for small torque transmission.
• 

Hence, the correct option is (A).