Which of the following cannot act as a Lewis base?

Which of the following cations is the most stable?

What is the difference between the bond angles in the cationic species of $PCl_5$ and $PBr_5$ in the solid state?

If two different non-axial $d-$orbitals having $xz$ nodal plane form $\pi-$bond by overlapping each other,then the internuclear axis will be:

Match the following:

List-$I$ (Bond)	List-$II$ (Bond enthalpy in $kJ \ mol^{-1}$)
$(A)$ $Si-Si$	$(I)$ $240$
$(B)$ $C-C$	$(II)$ $297$
$(C)$ $Sn-Sn$	$(III)$ $348$
$(D)$ $Ge-Ge$	$(IV)$ $260$

Fill in the blanks given below:
$(i)$ The two theories based on the principles of quantum mechanics are ........ and ......... .
$(ii)$ The electronic configuration of carbon in the excited state is ........... .
$(iii)$ Through the combination of atomic orbitals,....... are formed.
$(iv)$ The ${\rm{CH}}_4$ molecule has a .......... shape.