Explanation:

A modem (modulator-demodulator) is a device that converts digital signals from a computer into analog signals suitable for transmission over analog lines, such as telephone lines, and converts incoming analog signals back into digital data. Its primary function is to enable data communication over a network medium that is not natively digital. Therefore, a modem is typically connected to a 'Phone line' (for DSL or dial-up modems) or a cable line (for cable modems) to facilitate internet access. Option (a) 'Mouse' is an input device for user interaction. Option (b) 'Monitor' is an output display device. Option (d) 'Motherboard' is the main circuit board inside a computer that connects all internal components. While an internal modem might connect to the motherboard via a PCI/PCIe slot or a USB port, its external connection for data transmission to the internet service provider is to a communication line like a phone line.