The correct answer is: A. polling
Polling is a method of checking the status of a device or system by repeatedly sending a signal and waiting for a response. In distributed systems, polling is used to detect link and site failure by periodically sending a message to each node and waiting for a response. If a node does not respond, it is assumed to have failed.
Handshaking is a method of establishing a communication link between two devices. It involves a series of messages being exchanged between the devices to agree on the parameters of the communication, such as the data rate and error checking. Handshaking is not used to detect link and site failure.
Token passing is a method of controlling access to a shared resource. A token is passed from one device to another, and only the device that has the token can access the resource. Token passing is not used to detect link and site failure.
In conclusion, the correct answer is: A. polling