statement I : Down the group basic strength decreases from NH₃ to BiH₃

statement II : Down the group electron density decreases on central atom

General formula of 15^th group hydrides is

central atom contains a lone pair

Down the group , the size of the central atom increases,electron density on the central atom decreases.

So the tendency of donating electron pair decreases, basic strength decreases

(According to Lewis theory electron pair donars are bases)