Option 1: Incorrect. A modem is not a software. It is actually a hardware device that is used to connect a computer or other devices to the internet or a computer network.

Option 2: Incorrect. Modem does not help in stabilizing the voltage. Its main function is to transmit and receive data signals.

Option 3: Incorrect. Modem is not an operating system. An operating system is a software that manages computer hardware and software resources. A modem is a separate hardware device that is used in conjunction with an operating system.

Option 4: Correct. A modem is a device that converts analog signals (such as those transmitted over telephone lines) into digital signals that can be understood by computer devices, and vice versa. This conversion allows for data transfer between devices using different types of signals.