To invoke the LISP system, you must enter

To invoke the LISP system, you can use either the keyword AI or LISP, or you can enter CL (Common Lisp). These are different ways to initiate the LISP environment.
Option A: AI is one of the keywords used to invoke the LISP system.
Option B: LISP is another keyword used to invoke the LISP system.
Option C: CL (Common Lisp) is a third option to invoke the LISP system.
Therefore, the correct choice is Option D: both b and c, as both LISP and CL (Common Lisp) can be used to invoke the LISP system.