CBSE

In B.Com, you are going to have subjects like accounting and economics. You are going to study three types of economics in 3 years. Those are microeconomics, macroeconomics and international economics.

Hello Lekhika,
Yes, you can apply just for one subject in the improvement exam which can be clubbed with your previous marks if you score more than what you have scored previously.

Hello Vijay,
In the present system, I don't think you have an option of appearing for an additional paper in 12th or set of papers again! Also, what will you do? You would have become age barred.
Take up some graduation course.

Yes, you will get a new mark sheet.

You can try to get admission in VIT and SRM.

Yes, you do have a choice. The improvement exam form will release in July.
This is totally an online process so there is not any type of difficulty.
Your exam can be scheduled for next year with the board exams, without any practicals.

70% for PCMB in CBSE is not that bad. For you the career options are countless. You might go opt the following:
1. Engineering (Stream can be chosen according to your interest)
2. Medical Field (MBBS or BMS)
3. B.Sc. (Again stream to be chosen according to your interest)
4. Law (if interested)
Other than that if you want to choose any you can.
In the end you have to decide what your interest lies in.