Final answer:
The correct option to perform negative unity feedback for the overall system transfer function is 'feedback(sys, -1)' function.
Step-by-step explanation:
The correct option to perform negative unity feedback for the overall system transfer function is a) 'feedback(sys, -1)' function. Negative feedback is a mechanism used in control systems where the output of the system is fed back to the input with a negative sign. It helps in stabilizing the system and reducing errors or disturbances. In the context of system transfer functions, the 'feedback()' function with a negative sign as the argument accomplishes this negative unity feedback.