why nda is applicable for boys only

National Defense Academy is only for boys.This may seem a bit harsh but the trend followed by the NDA is similar to the RIMC and other academic establishments of the defense services.Indian Navy has finally given it's consent to offer permanent commission to women, which means girls can not only join the Indian Navy as an officer, but also endure and flourish in the service throughout her life. 
Any girls can not join the Navy through the NDA but certainly there are other options open to her. Visit Navy official website at:
http://www.joinindiannavy.gov.in
for more information.