Which type of memory retains data even when the computer is powered off?

Read-Only Memory (ROM) is a type of non-volatile memory, meaning it retains data even when the computer is powered off. This characteristic of ROM is crucial for storing firmware, which is the software permanently programmed into a hardware device that provides the necessary instructions for booting the system and performing hardware initialization.

ROM is often used to store the system's BIOS (Basic Input/Output System), which is essential for starting the computer and enables it to communicate with other hardware components before the operating system loads. Because of its non-volatile nature, ROM does not require power to maintain the stored information, making it reliable for critical system functions.

In contrast, other types of memory listed in the question, such as Random Access Memory (RAM), are volatile, meaning they lose their data when the power is turned off. Flash storage, while also non-volatile like ROM, is primarily used for data storage and is not categorized under the same classification as ROM. Cache memory, used to speed up data access for the CPU, is another form of volatile memory. Thus, ROM specifically stands out as the correct answer in this context due to its role and functionality in retaining information without power.