Engineering

This would depend a lot on your personality traits & interest areas. Each MBA specialisation needs specific personality traits for excelling in the profession say if you are outspoken and like traveling, meeting people, take target pressure. Marketing could be a preference and if you are a people's person wherein you like counseling people and act as an advisory & want a desk top job bonded by rules & regulations then HRM will be exciting so do evaluate your inherent personality traits and then take a call. Since you are a B.Tech in fire & safety and in case you are interested in operations then specialisation in operations should be preferred.

It depends upon your age.
NDA 2019 Eligibility- Qualification
The academic qualifications varies slightly according to the post that the candidate has applied for. The details of educational qualification for different posts are specified below:
For Army Wing of National Defence Academy: Pass certificate in 10+2 exams via recognised board or a University. For Air Force and Naval Wings of National Defence Academy and for the 10+2 Cadet Entry Scheme at the Indian Naval Academy: Candidate should have passed Class 12th exam in the 10+2 pattern of School Education or equivalent with Physics and Mathematics conducted by a State Education Board or a University.