Answer
$PH_3 < NH_3 < KCl
Work Step by Step
$MgO$ has highest boiling point due to its ionic bonding with $+2$ charge.
$KCl$ also has ionic bonding but only $+1$ charge.
$NH_3$ has $N-H$ bond that can form $H$ bonding, which is weaker than ionic bonding.
$PH_3$ does not have $H$ bonding or ionic bonding so it has lowest boiling point.
Boiling point in increasing order:
$PH_3 < NH_3 < KCl
