Modifier keys are special keys on a computer keyboard that modify the normal action of another key when pressed together. They are commonly used to perform shortcuts and execute commands in various applications.

The correct answer is option 'D' - All of the above. All three options - Alt, Ctrl, and Windows - are examples of modifier keys.

Below is an explanation of each modifier key:

1. Alt:
- The Alt key, short for "Alternate," is one of the modifier keys found on computer keyboards.
- When pressed in combination with another key, it performs a specific function or command.
- For example, Alt + Tab is commonly used to switch between open windows or applications.
- Alt is often used in combination with other keys to access menu items or perform actions within software applications.

2. Ctrl:
- The Ctrl key, short for "Control," is another modifier key found on computer keyboards.
- It is primarily used to perform keyboard shortcuts or hotkeys.
- For example, Ctrl + C is used to copy selected content, and Ctrl + V is used to paste it.
- Ctrl is widely used in combination with other keys to perform various functions, such as saving files, opening new tabs in web browsers, and undoing or redoing actions.

3. Windows:
- The Windows key is a modifier key found on keyboards designed for Microsoft Windows operating systems.
- It typically features a Windows logo and is located between the Ctrl and Alt keys.
- Pressing the Windows key alone opens the Start menu or the Start screen, depending on the Windows version.
- The Windows key is often used in combination with other keys to perform system-level functions, such as launching applications, accessing system settings, or managing virtual desktops.

In summary, all three options - Alt, Ctrl, and Windows - are modifier keys that modify the normal action of other keys when pressed together. They are essential for executing shortcuts, accessing menu items, and performing various functions within software applications.