The correct answer is: A. is usually the primary user interface.
A command interpreter is a program that takes a command as input and executes it. It is usually the primary user interface for a computer system, as it allows users to interact with the system by typing commands.
Option B is incorrect because command interpreters do not always require fixed format commands. For example, the Bash shell allows users to enter commands in a variety of formats, including free-form, positional, and named parameters.
Option C is incorrect because command interpreters are not always menu-driven. For example, the Bash shell does not have a menu interface.
Option D is incorrect because command interpreters are not always quite different from the SCL interpreter. For example, the Bash shell and the SCL interpreter are both command interpreters that take commands as input and execute them.
Option E is incorrect because the correct answer is A.