Loader is the correct option. Let's discuss in detail why it is the correct answer.

System Software
System software is a type of software that provides a platform for other software to run on. It includes various programs and routines that help in managing the computer hardware and software resources. System software is essential for the proper functioning of a computer system.

Main Memory
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 computer system. It provides fast access to data and instructions for the CPU (Central Processing Unit).

Residence in Main Memory
When a computer system is running, it loads various programs and data into the main memory for execution. The main memory is the primary storage location for the programs and data that are currently in use. However, not all software resides in the main memory at all times.

Text Editor
A text editor is a type of software that is used for creating and editing text files. It allows users to write and modify text-based documents. Text editors are not system software and do not reside in the main memory at all times.

Assembler
An assembler is a type of program that converts assembly language code into machine code. It is used to create executable programs from assembly language source files. Assemblers are not system software and do not reside in the main memory at all times.

Linker
A linker is a program that combines multiple object files and libraries to create an executable program. It resolves references between different parts of the program and generates the final executable file. Linkers are not system software and do not reside in the main memory at all times.

Loader
A loader is a program that loads executable programs into the main memory for execution. It is responsible for transferring the program from storage (such as a hard disk) into the main memory. The loader prepares the program for execution by allocating memory space, resolving external references, and initializing various data structures. Loaders are essential system software and always reside in the main memory when they are in use.

Therefore, among the given options, only the loader resides in the main memory always.