Answer:
The taskbar is an element of an operating system located at the bottom of the screen. It allows you to locate and launch programs through Start and the Start menu, or view any program that's currently open.
The answer is 2-to-the-power-of-n, since for every input, the number of different combinations doubles. From your list I think answer A is meant to indicate 2ⁿ.
Most significant digit. That's the same that it's called for any number system.