Which compound has the least ionic character?

Question

1.) KI 2.)HCl 3.) MgS 4.) NO

Answer 1

Degree of ionic character can be determined based on differences in electronegativity.  

See here: http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Electronegativity-861.html

The smallest difference in your list of 4 would be between Nitorgen (N) and Oxygen (O), with a difference of 3.44 - 3.04 = 0.40 (all other pairs in your list have larger differences)

Answer 2

Answer: 4.) NO since N and O are both non metals, and have close electronegativities.