Which of the following is least ionic?

Give a reason for the following:

Ionic compounds have a high melting point.

Two elements $\mathrm{A}$ and $\mathrm{B}$ have electronegativities $1.2$ and $3.0$ respectively. The nature of bond between $\mathrm{A}$ and $\mathrm{B}$ would be

Find out the correct order of ionic character in the following molecules

$\left(\mathrm{i}\right) {\mathrm{SO}}_2$     $\left(\mathrm{ii}\right) {\mathrm{K}}_2\mathrm{O}$     $\left(\mathrm{iii}\right) {\mathrm{N}}_2$    $\left(\mathrm{iv}\right) \mathrm{LiF}$