Storage is a mechanism that enables a computer to retain data, either temporarily or permanently. Storage devices such as flash drives and hard disks are a fundamental component of most digital devices since they allow users to preserve all kinds of information such as videos, documents, pictures and raw data. Important PointsHard Disk : A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electromechanical data storage device that stores and retrieves digital data using magnetic storage and one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Main Memory : The main memory in a computer is called Random Access Memory. It is also known as RAM. This is the part of the computer that stores operating system software, software applications, and other information for the central processing unit (CPU) to have fast and direct access when needed to perform tasks. It is called "random access" because the CPU can go directly to any section of main memory, and does not have to go about the process in sequential order. CD-ROM: CD-ROM is a pre-pressed optical compact disc that contains data. Computers can read€”but not write or erase€”CD-ROMs, i. e. it is a type of read-only memory. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data is only usable on a computer. Hence the correct order of the computer storage devices in order of speed of data retrieval from fastest to slowest is as follows: main memory, hard disk, and CD-ROM. Therefore (B), (A), and (C) is the correct answer.