Which one of the following is most acidic?

What is the correct order of the first electron gain enthalpy for $O, S,$ and $Se$?

The correct sequence of decreasing number of $\pi$-bonds in the structures of $H_2SO_3$,$H_2SO_4$,and $H_2S_2O_7$ is:

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$ : Both rhombic and monoclinic sulphur exist as $S_8$ while oxygen exists as $O_2$.
Reason $(R)$ : Oxygen forms $p \pi-p \pi$ multiple bonds with itself and other elements having small size and high electronegativity like $C, N$,which is not possible for sulphur.
In the light of the above statements,choose the most appropriate answer from the options given below:

The laboratory test 'tailing of mercury' is used to identify:

Explain the nature of oxides of group-$16$ elements. Write a note on halide compounds of group-$16$ elements. Write a note on hydride compounds of group-$16$ elements.