Which one of the following molecules has a coordinate bond?

Which of the following compounds contains a coordinate covalent bond?

How many hydrogen atoms are bonded to the nitrogen atom in an ammonium ion $(NH_4^+)$?

Bonds present in $N_2O_5$ (nitrogen pentaoxide) are

In $BF_3$,the $B-F$ bond length is $1.3 \ \mathring{A}$. When $BF_3$ reacts with $Me_3N$,it forms an adduct $Me_3N \to BF_3$. In this adduct,the $B-F$ bond length will be ..........

Which of the following compounds forms a dative bond with ammonia?