Secondary memory, also known as external memory or auxiliary memory, refers to storage devices that are used to store data for long-term use. Unlike primary memory (RAM and ROM), which is volatile and temporary, secondary memory retains data even when the power is turned off.

Primary memory is used for immediate data access and processing, while secondary memory is used for long-term storage and retrieval of data. It is typically larger in capacity and slower in speed compared to primary memory.

Secondary memory is an essential component of computer systems as it provides a means to store and retrieve data when needed. It is commonly used for storing operating systems, applications, files, and other data that can be accessed later.

The options provided in the question are as follows:

a) CD, DVD, and flash drives: These are examples of secondary memory devices. CDs and DVDs are optical storage media that use lasers to read and write data. Flash drives, also known as USB drives or thumb drives, use flash memory technology to store data.

b) RAM and hard disk: RAM (Random Access Memory) is a type of primary memory that is used for temporary storage of data and instructions while a computer is running. A hard disk, on the other hand, is a secondary memory device that uses magnetic storage to store data.

c) ROM, floppy disk, and flash drives: ROM (Read-Only Memory) is a type of primary memory that stores permanent instructions and data that cannot be modified. Floppy disks, although not commonly used anymore, were a popular form of portable secondary memory in the past. Flash drives, as mentioned earlier, are also examples of secondary memory devices.

d) RAM and ROM: These are both types of primary memory and not secondary memory.

In conclusion, the correct option is a) CD, DVD, and flash drives, as they are examples of secondary memory devices used for long-term storage and retrieval of data.