Hello Anwar Naseem, you can't give the improvement exam for a particular subject in HSC science. As to give the improvement exam you have to give all the exams again. 

it is a rule from HSC science that you can't give the improvement exam for only one subject.

Hello Anwar, A candidate obtaining the lowermost Grades E1 and E2 in any or all the five subjects (excluding 6th subject) under Scholastic Area A at the exams conducted by the education board or school is eligible for the Improvement Exam for any or all of the five subjects. 

But a candidate can only appear in the Improvement Exam held during the month of July of that particular year, not any other subsequent year in future.

Check out CBSE's official site. Hope this helps.