Which mechanical part or feature listed in the section on critical vehicle systems do you think is most important? why is that particular part or feature so important?
how do you benefit from that part or feature?

(1) The mechanical part or feature listed in the section on Critical Vehicle Systems do you think is most important are steering system and braking system. 
(2) That particular part or feature so important because a steering system, in good mechanical order, is essential in maintaining control of the vehicle. In an emergency, it is most critical to be able to stop the vehicle within a short distance, therefore the brakes, are most critical.
(3) The function itself expresses the benefits that it brings.