Loader

The correct answer is option 'D', Loader. The loader is a system software that resides in the main memory always. Let's understand why the loader is the correct answer.

System Software
System software refers to a collection of programs that manage and control the computer hardware and provide a platform for running application software. It includes operating systems, device drivers, utility programs, and loaders, among others.

Loader
A loader is a system software responsible for loading executable programs into the main memory for execution. When a program is compiled and linked, it is in the form of an object file, which contains machine code instructions and data. The loader is responsible for taking this object file and loading it into the main memory so that the CPU can execute it.

Residing in Main Memory
The main memory, also known as primary memory or RAM (Random Access Memory), is a volatile memory that stores data and instructions that are currently being used by the CPU. The main memory is directly accessible by the CPU and is much faster than secondary storage devices like hard drives.

The loader, being a system software, needs to be always available in the main memory to perform its tasks efficiently. It is loaded into the main memory during the booting process and remains there as long as the system is running. This allows the loader to handle the loading of multiple programs into memory without the need for repeated loading of the loader itself.

Other Options
Let's briefly discuss why the other options are not correct:

- Text Editor: A text editor is an application software used for creating and editing text files. It is not a system software and is not required to reside in the main memory always.
- Assembler: An assembler is a program that converts assembly language code into machine code. It is used during the compilation process, but it is not required to reside in the main memory always.
- Linker: A linker is a program that combines multiple object files and libraries to create an executable program. It is also used during the compilation process, but it is not required to reside in the main memory always.

Conclusion
In conclusion, the loader is the system software that resides in the main memory always. It is responsible for loading executable programs into the main memory for execution. The other options, such as text editor, assembler, and linker, are not system software that need to reside in the main memory always.