Which of the following pairs of substances cannot exist together in a solution?

Which one of the following statements is correct?

The dissociation constant of pure water at $18\,^oC$ will be .......... (Degree of dissociation of pure water at $18\,^oC = 1.8 \times 10^{-9}$)

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$2NaOH + Zn(OH)_2 \rightarrow Na_2ZnO_2 + 2H_2O$

By adding $20 \ mL$ $0.1 \ N$ $HCl$ to $20 \ mL$ $0.1 \ N$ $KOH$,the $pH$ of the obtained solution will be

The conductivity of a weak acid $HA$ of concentration $0.001 \, mol \, L^{-1}$ is $2.0 \times 10^{-5} \, S \, cm^{-1} .$ If $\Lambda_{m}^{\circ} (HA)=190 \, S \, cm^{2} \, mol^{-1}$,the ionization constant $(K_{a})$ of $HA$ is equal to $....\, \times 10^{-6} .$ (Round off to the Nearest Integer)