A printer is considered a traditional input and output device because it receives data (input) from the computer and produces a physical output, such as a printed document. Traditional input/output (I/O) devices are peripherals that interact with the computer to send or receive information. While a keyboard is an input device and a monitor is an output device, the printer serves as a device that translates digital files into physical media.
A hard drive is a storage device, not traditionally classified as input/output.
A keyboard is solely an input device.
A display monitor is solely an output device.
Reference:
CompTIA A+ Exam Objectives (Core 1)
Official CompTIA IT Fundamentals (ITF+) Certification Guide