Hi Ashish,

No, BITS does not accept partial improvement.

Yes, they do accept improvement exam marks but you will have to give the improvement exam of all the subjects that you studied in the 12th Boards.

Also, BITS only accepts the latest improvement marks, for example, if you gave the 12th Board in 2017, then gave improvement exam in 2018, then only your 2018 marks are valid otherwise not.

Hope this helps you.

All the best!!!