Which of the following compounds is chiral?

Determine the correct priority order for the following compound to assign the $R$ or $S$ configuration.
$\begin{array}{c} CHO \\ | \\ H-C-OH \\ | \\ CH_2OH \end{array}$

Which of the following is chiral?

The similarity between geometric and optical isomerism is that .....

The correct statements with respect to the above pair of reactions are that:
$(I)$ The reactions are stereospecific.
$(II)$ $(X)$ is erythro and $(Y)$ is threo isomer.
$(III)$ $(X)$ is threo and $(Y)$ is erythro isomer.
$(IV)$ Each of $(P)$ and $(Q)$ gives a mixture of $(X)$ and $(Y)$.

Arrange the following compounds in decreasing order of their acidic strength: