Handshake signals are used in computer networks to establish a connection between two devices. They ensure that both devices are ready to communicate and understand each other's instructions. Handshake signals help in synchronizing the communication process and are essential for error-free data transmission.

The given options are:

a) START
b) STOP
c) ACKNOWLEDGE
d) START and STOP

Among these options, the correct answer is option 'C', which is ACKNOWLEDGE.

Now, let's understand each of these options and determine whether they are handshake signals or not.

a) START:

The START signal is not specifically a handshake signal. It is used to indicate the beginning of a transmission or a data block. It is not essential for establishing a connection between devices. Therefore, it is not a handshake signal.

b) STOP:

Similar to the START signal, the STOP signal is not a handshake signal. It is used to indicate the end of a transmission or a data block. It does not play a role in establishing a connection between devices. Therefore, it is not a handshake signal.

c) ACKNOWLEDGE:

The ACKNOWLEDGE signal is indeed a handshake signal. It is used to acknowledge the receipt of a message or data block. When a device receives data from another device, it sends an ACKNOWLEDGE signal to confirm that it has received the data correctly. This signal ensures that both devices are synchronized and ready for further communication. Therefore, ACKNOWLEDGE is a handshake signal.

d) START and STOP:

The combination of START and STOP signals does not specifically represent a handshake signal. While both signals are used in data transmission, they do not serve the purpose of establishing a connection or synchronizing the devices. Instead, they mark the beginning and end of a data block. Therefore, START and STOP together are not considered handshake signals.

In conclusion, the only handshake signal among the given options is ACKNOWLEDGE (option 'C'). It is used to acknowledge the receipt of data and synchronize the communication between devices.