(a)The tendency to act as lewis base decreases from NI3 to NF3 .
(b)The Stability Of sulphides of group-15 elements increases down the group..
Trihalides are basic due to the tendency to donate lone pair of electrons on nitrogen atom.
Now, the electronegativity of halogens follows the order F> Cl> Br>I
Therefore, tendency of nitrogen halides to donate electrons decreases in the order NI3> NBr3> NCl3 >NF3.
Therefore, the basic character or tendency to act as a Lewis base decreases from NI3 to NF3.