Is it possible to submit the final report after 31st december? because i have tough schedule so it is not possible to complete the course,so is it valid to submit the project after 31st dec. 2015? or else i will have to enroll to cs50 next year?
No, it is not possible to submit Your final project after December 31st because it will be past the deadline already. However, you can enroll again next year and resubmit all your psets (which I believe you've done already) and your final project. Some psets might change though but I do not have certainty of that.