CBSE 12th

Please write about the course name for better response. However the eligibility criteria of GIBS's BBA course is given below. ELIGIBILITY CRITERIA BBA Pass in 12th class of 10+2 of CBSE or equivalent with a minimum of 50% marks in aggregate and must also have passed English (core or elective or functional) as a subject. Aggregate of 50% marks in the 12th class for the purpose of eligibility will be taken as the aggregate of best four subjects including English and compulsory subject (s), if any. So you can be eligible for BBA if meeting the above criteria.

PG in archaeology or Diploma.

Yes you can join NIOS for only one subject. NIOS is offering On Demand Examination, under this The learner has also the option to appear under the On-Demand Examination System (ODES) of NIOS at the Secondary and the Senior Secondary levels at the NIOS Headquarters, NOIDA, at the Regional Centres of NIOS, and at designated Kendriya Vidyalayas across India. Please read NIOS prospectus for more details
https://www.nios.ac.in/media/documents/prospectus/2020/Academic_Prospectus_2020_21.pdf.