The correct answer is Firmware.

• Firmware
  • In electronic systems and computing, it is a tangible electronic component with embedded software instructions, such as a BIOS. 
  • It provides the necessary instructions for how the device communicates with the other computer hardware.
  • Typical examples of devices containing firmware are embedded systems (such as traffic lights, consumer appliances, and digital watches), computers, computer peripherals, mobile phones, and digital cameras.
• ROM: Read-Only Memory
  • It refers to computer memory chips containing permanent or semi-permanent data.
  • It is non-volatile.
  • The data remains intact even when the power is switched off.
  • It contains the programming needed to start a PC, which is essential for boot-up.
  • It performs major input/output tasks and holds programs or software instructions.
  • It is the permanent memory built into your computer.
• Hard Disk
  • It is the rigid magnetic disc that stores data.
  • It is located within a drive unit.
  • It is a non-volatile storage device that contains platters and magnetic disks rotating at high speeds.
• Main memory refers to physical memory that is internal to the computer. It is where programs and data are kept when the processor is actively using them.
• Since firmware is not a memory and the other three options are, firmware is the odd one out.