HI aspirant

For NDA exams minimum education qualification required to apply for NDA is 10+2 any subjects (Army), 10+2 (Physics and Maths for Air Force & Navy). To know more about eligibity criteria for NDA refer this article

https://competition.careers360.com/articles/nda-eligibility-criteria

Hello

Yes you are eligible only for army wing. Selection criteria for air force and Navy wing is you must have PCM in 12th . But for army wing you have pass certificate of 10 + 2 via recognized board or university. National Defence academy conducts the Entrance exam twice a  year for all the three wings. The syllabus consists of two subjects Maths and GAT(General ability test). Maths(300 marks) is of 11th and 12th level and GAT (600 marks)  consists subjects like English(200 marks) ,General knowledge,Current affairs, Biology , History, Geography, Political Science, Economics.

Yes a biology 12th student is eligible for NDA.

For NDA exam you must have 50% marks in 12th. if Biology in your 12Th examination then you are eligible to apply for Army Wings of NDA. Age limit for this exam is 16.5 to 19 Years

For Naval and Air Force Wings, You have to complete your 12Th examination with Maths and Physics as a compulsory Subjects.To apply for this examination your age should between 16.5 to 19 Years.