Explanation

(a)
$XeO_{4}$ is formed by promoting one $5s$ and there $5p$-electrons of $Xe$ to higher energy. $5d$ orbitals giving eight unpaired orbitals hybridize to give $sp^{3}$ hybridisation which form sigma bonds with four O atoms. The four unhybridised singly occupied $5d$ orbitals form four $p\pi - d\pi$ bonds with oxygen atoms