Why Phosphorus has d orbital?

Phosphorous,z:15
Electronic configuration of Phosphorus is 1s² 2s² 2p⁶ 3s² 3p³ 3d⁰.
phosphorus belongs to third period and its principal quantum number or energy level is 3 which contains 3s, 3p, and 3d orbitals.

The type of orbitals depend on the shell. In 1st shell, there is only s-orbital as in Hydrogen and Helium. In 2nd shell there are only p and s orbitals. d-orbital comes in third shell. therefore phosphorous has vacant d orbital as 3d exists. From Phosphorus onwards all the elements of that group have d-orbitals. Phosphorus can unpair all it's electrons to the d-orbital, giving it a maximum valency of 5, as seen in PF5. Same with chlorine, which gives it a maximum valency of 7. This unpairing happens in excited states.