In order to help the student expand his/her knowledge I will help answer the question. This in hope that the student will get a piece of knowledge that will help him/her through his/her homework or future tests.
A professional is one of the best individual resources to help someone quit smoking. It will not only have a better chance of success but it makes the individual feel more sure about himself with the help of someone professional. The correct choice is letter
a. a professional
I hope it helps, Regards.