Arrange the following in the correct order of their acidic strength.

I. $\mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2$

II. $\mathrm{CH} \equiv \mathrm{CH}$

III. $\mathrm{CH}_3-\mathrm{C} \equiv \mathrm{CH}$

IV. $\mathrm{CH}_3-\mathrm{CH}_3$

$$ \mathrm{C}_3 \mathrm{H}_4+\mathrm{H}_2 \mathrm{O} \xrightarrow[333 \mathrm{~K}]{\mathrm{Hg}^{2+} / \mathrm{H}^{+}}[X] \stackrel{\text { Tautomerisation }}{\rightleftharpoons} Y $$

$X$ and $Y$ are respectively

Addition of HBr to propene in presence of a peroxide takes place contrary to Markownikoff rule. This can be explained by the mechanism involving

The rate of attack of an electrophile is least when $X$ in the given compound is