Which of the following is a conjugate acid-base pair?

The strongest Bronsted base among the following anions is:

The conjugate base of phosphorus acid is $x$. The conjugate base of oleum is $y$. What are $x$ and $y$,respectively?

The $K_{a}$ values of $A$,$B$,and $C$ are $1.8 \times 10^{-4}$,$5 \times 10^{-10}$,and $3 \times 10^{-8}$ respectively. The correct order of their acidic strength is

Select the reaction which will be favoured in the forward direction.

Which of the following is the strongest conjugate base?