The correct answer is C. Communicate with other computer peripherals.
A computer port is a physical interface through which a computer can connect to other devices. Ports can be used to connect input devices, such as keyboards and mice, output devices, such as monitors and printers, and storage devices, such as hard drives and optical drives.
Option A is incorrect because hard disks are internal devices that are not connected to the computer through a port.
Option B is incorrect because downloading files is a process that is typically done through a web browser or another application, not through a port.
Option D is incorrect because computer ports are used to communicate with other computer peripherals.