In which compound is the hydrogen bonding the strongest in its liquid phase?

Identify which molecule in each of the following pairs has stronger $H$-bonding:
$(i)$ $H_2O$ and $H_2S$
$(ii)$ $NH_3$ and $PH_3$
$(iii)$ $HF$ and $HCl$

The boiling points of methanol,water,and dimethyl ether are respectively $65\,^{\circ}C$,$100\,^{\circ}C$,and $-24.8\,^{\circ}C$ (note: dimethyl ether is a gas at room temperature). Which of the following best explains these wide variations in boiling point $(b.p.)$?

The melting point of $o$-hydroxybenzaldehyde $(A)$ is lower than that of $p$-hydroxybenzaldehyde $(B)$. This is because

Hydrogen bonding is present in:

The molar enthalpy of vaporisation of acetone is less than that of water. Why?