It is necessary to study all the chapters from the book you are using for NEET. If you may to keep some chapters for option, atleast go through is once before the exam as even that may help if required. I suggest you not to leave any chapter. Appear for mock tests as you may go through some questions from the chapters that you may have left or kept for option. Make use of books which has explained everything in normal language which can be understood by you. So it is up to you to leave any chapter or study all as the score will decide the college you will get for higher studies.