Explanation:

This question asks to identify the odd one out from the given list of computer components based on their classification. Option (a) 'Hard disk' is an internal storage device used for long-term data retention. Option (b) 'Motherboard' is the main circuit board that houses and connects all other internal components, acting as the central nervous system of the computer. Option (c) 'RAM' (Random Access Memory) is an internal volatile memory module used for temporary storage of data that the CPU is actively using. All three, Hard disk, Motherboard, and RAM, are internal components crucial for the computer's operation and are typically found inside the computer case. Option (d) 'Mouse' is an external peripheral input device used for interacting with the computer's graphical user interface. Therefore, the 'Mouse' is the odd one out as it is an external peripheral, while the others are internal core components of the computer system.