Since you have 75% in Class 12, even if you do not take an imporvement exam wont effect your career. Your main target should be to secure admission in a Eminent Insitute for Graduation followed by Masters and Doctoral Degree for a career in research and academia. You need to be serious with your courses and practical projects at each stage of your student life from now on, be it while your Undergraduate or Postgraduate. Scientists and Researchers have an excellent Reading skills and they finish reviewing Journals and Texts very quickly and in an informative way. You need to practice this skills. Study all your course books, as well as distinguished books by foreign authors in details during your college life.