To invoke the LISP system, you must enter

Answer: Option D

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.