Q.The stabilisation of a half filled d-orbital is more pronounced than that of the p-orbital. Why?

Exactly half-filled ($p^3$, $d^5$, $f^7$) and exactly completely-filled ($p^6$, $d^{10}$, $f^{14}$) sets of degenerate orbitals are more stable than any other partial filling of the same set, for two reinforcing reasons.

Symmetrical distribution. In both the half-filled and fully-filled cases, every orbital of the degenerate set is treated identically - all singly occupied with parallel spin, or all fully paired - giving a perfectly symmetrical electron distribution. Because the individual orbitals of a degenerate set (such as $p_x,p_y,p_z$) point in different spatial directions, electrons in different orbitals shield each other from the nucleus only weakly; less mutual shielding means a stronger net (effective) nuclear attraction on each electron, and therefore greater stability. …