A taper key which fits half in the key way of the hub and half in the key way of shaft, is known as

In machine design, keys are used to connect rotating machine elements such as shafts and hubs to transmit torque. A sunk key is a type of key that is rectangular or square in shape and fits completely within the keyway of both the shaft and the hub. It is "sunk" or fully inserted into the keyway, providing a secure connection between the two components.

Among the options provided, the correct answer is Option A: sunk key. This type of key is commonly used to prevent relative motion between the shaft and the hub while transmitting torque.

Therefore, the correct answer is Option A: sunk key.