Which of the following compounds forms the maximum number of hydrogen bonds?

Use the information and data given below to answer the questions $(a)$ to $(c)$. Stronger intermolecular forces result in higher boiling point.
Strength of London forces increases with the number of electrons in the molecule.
Boiling points of $HF, HCl, HBr$ and $HI$ are $293 \ K, 189 \ K, 200 \ K$ and $238 \ K$ respectively.
$(a)$ Which type of intermolecular forces are present in the molecules $HF, HCl, HBr$ and $HI$?
$(b)$ Looking at the trend of boiling points of $HCl, HBr$ and $HI$,explain out of dipole-dipole interaction and London interaction,which one is predominant here.
$(c)$ Why is the boiling point of hydrogen fluoride highest while that of hydrogen chloride lowest?

The hydrides of the first elements in groups $15-17$, namely $NH_3$, $H_2O$ and $HF$ respectively show abnormally high values for melting and boiling points. This is due to

The boiling point of $NH_3$ is much higher than that of $PH_3$ because.......

The molecule which does not exhibit strong hydrogen bonding is

$NH_3$ has a much higher boiling point than $PH_3$ because