Explanation:

A "Disk drive" (such as a Hard Disk Drive or Solid State Drive) is the primary component in a computer responsible for persistent storage of digital information, including the operating system, applications, and user data. Option (b) "Keyboard" is an input device used to enter text and commands. Option (c) "Monitor" is an output device that displays visual information. Option (d) "Printer" is an output device that produces hard copies of digital documents. None of these other options are designed for storing information. Therefore, (a) Disk drive is the correct answer as its fundamental function is data storage.