A student studies four species: NH₄⁺, NCl₃, PCl₅ and N₂O₅…

Step 1. Oxidation state. In $\text{N}_{2}\text{O}_{5}$, the five oxygen atoms give $5 \times (-2) = -10$. The molecule is neutral, so the two nitrogen atoms give $+10$. Each has $+5$.

Step 2. Bonds on nitrogen. The question gives four, so the covalency of nitrogen is 4, but its oxidation state is $+5$. These are not the same.

Step 3. Bonds on phosphorus. Each chlorine atom is joined by one bond, so phosphorus in $\text{PCl}_{5}$ has 5 bonds.

Step 4. Why not $\text{NCl}_{5}$. Nitrogen has no d orbitals in its second shell and cannot hold more than eight electrons, so its covalency cannot go beyond 4.