The correct answer is (a) Modem.
A modem is a device that modulates and demodulates signals. It is used to convert digital signals into analog signals that can be transmitted over telephone lines, and vice versa. This allows computers to communicate with each other over the telephone network.
An analog signal is a continuous signal that can take on any value within a given range. A digital signal is a discrete signal that can only take on a finite number of values.
A monitor is a device that displays information from a computer. It can be a cathode ray tube (CRT) monitor, a liquid crystal display (LCD) monitor, or a plasma display.
An O.C.R. (optical character recognition) is a device that converts printed text into digital text. It is used to scan documents and convert them into files that can be edited or stored on a computer.
In conclusion, the correct answer is (a) Modem. A modem is a device that modulates and demodulates signals. It is used to convert digital signals into analog signals that can be transmitted over telephone lines, and vice versa. This allows computers to communicate with each other over the telephone network.