If you are taking a drop year and planning to apply again with improved marks in one subject, then yes, most colleges and universities do accept improvement marks from recognized boards like CBSE, ICSE, or state boards. Your new marksheet will carry the updated score for that subject, and during admission, the latest marksheet is usually considered.

However, you must check the admission rules of the specific institute or university where you are applying. Some institutes may ask for the complete set of marks (original plus improvement), while others will directly consider the improved marks as final.