Among the following which compounds conjugate base is strongest in comparision to other given in the option?

The strength of the conjugate bases is in the order A<B<C<D
${\mathrm{NH}}_3$ is very weakest acid among all According to Bronsted-Lowry concept of acid-base,  conjugate base of weakest acid is very strong and vice-versa. Moreover, we can also say that ammonia is a base as per Lewis, hence it must be a weak acid. There fore the conjugate base is very strong  ${\mathrm{NH}}_2^{-1}$.