Explanation:

Ans: D
Two operating modes of AT are Real mode, protected mode.

Real Mode:

The original IBM PC could only address 1 MB of system memory, and the original versions of DOS created to work on it were designed with this in mind. DOS is by its nature a single-tasking operating system, meaning it can only handle one program running at a time. The decisions made in these early days have carried forward until now, and in each new processor, care had to be taken to be able to put the processor in a mode that would be compatible with the original Intel 8088 chip. This is called real mode.

Protected Mode:

Starting with the 80286 chip in the IBM AT, a new processor mode was introduced called protected mode. This is a much more powerful mode of operation than real mode, and is used in all modern multitasking operating systems.