Question

For nitrogen phosphorus + 5 oxidation state is more stable than + 3 oxidation state

Answer 1

Your statement is wrong.
Whenever,
lower oxidation state will be stable than higher oxidation state as we moving down the group.

this is because ,
The inert pari effect play a major role.
on the other hand,
in nitrogen there is no d orbital. we moving down the group , there is a vacant p orbital. so lower oxidation state only stable.

so +3 is stable than +5.