Which of the following is non-ionizable?

$A$ brown ring is formed in the ring test for $NO_{3}^{-}$ ion. It is due to the formation of :-

Which of the following complexes has the highest $C-O$ bond length?

Which of the following ligands can form both high-spin and low-spin complexes?

The number of moles of $NH_{3}$ that must be added to $2 \, L$ of $0.80 \, M \, AgNO_{3}$ in order to reduce the concentration of $Ag^{+}$ ions to $5.0 \times 10^{-8} \, M$ ($K_{\text{formation}}$ for $[Ag(NH_{3})_{2}]^{+} = 1.0 \times 10^{8}$) is ...... . (Nearest integer)
[Assume no volume change on adding $NH_{3}$ ]

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$NiCl_2 + NH_3$ (Excess) $\longrightarrow [Ni(NH_3)_6]^{2+}$