Introduction:
The mouse is an essential input device used to navigate and interact with graphical user interfaces (GUIs) on a computer. It provides users with a way to control the cursor on the screen and perform various tasks efficiently. In this response, we will discuss the statements given and explain why option 'D' - "All of these" is the correct answer.

Explanation:
Let's evaluate each statement and understand why it is correct.

a) It is a hand-operated device:
The mouse is a hand-operated device that is used by individuals to control the movement of the cursor on the computer screen. By moving the mouse on a flat surface, the user can control the position of the cursor, allowing them to interact with various elements and perform actions.

b) It sits outside the computer case:
Unlike internal components such as the motherboard, CPU, or hard drive, the mouse is an external device that sits outside the computer case. It is connected to the computer via a cable or wirelessly, allowing users to operate the mouse from a comfortable position outside the case.

c) It comes in many shapes and sizes:
Mice come in various shapes and sizes to cater to different user preferences and needs. They can range from compact and portable designs to ergonomic models that provide enhanced comfort during prolonged use. Some mice are specialized for gaming, while others are designed for general everyday use.

d) All of these:
Considering the explanations provided above, it is evident that all the statements (a, b, and c) are correct. The mouse is a hand-operated device that sits outside the computer case, and it comes in various shapes and sizes to suit different user requirements.

Conclusion:
In conclusion, the correct statement about the mouse is option 'D' - "All of these." The mouse is a hand-operated device that sits outside the computer case and comes in many shapes and sizes. It is an essential tool for navigating and interacting with graphical user interfaces on a computer.