The correct answer is C. Assembler language is a mnemonic form of machine language.
Assembler language is a low-level programming language that is used to create machine code. It is a mnemonic form of machine language, which means that it uses symbols to represent the instructions that the computer will execute. Assembler language is typically used to create programs that need to be very efficient, such as operating systems and device drivers.
Option A is incorrect because the primary user interface is typically a graphical user interface (GUI).
Option B is incorrect because assembler language does not require fixed-format commands.
Option D is incorrect because assembler language is similar to the SCL interpreter in that they both allow programmers to create machine code. However, they are different in that assembler language is a low-level programming language, while the SCL interpreter is a high-level programming language.
Option E is incorrect because C is the correct answer.