Ionic character depends on the electronegativity difference between the bonding elements. The greatest the elctronegativity diffference the greatest the ionic character.

Here in the given options, H-F has has the greatest ionic character. The electronegativity of Hydrogen is 2.1 and that of Fluorine is 4. Now the electronegativity difference is 4 - 2.1 = 1.9.

Electronegativity measures an element's ability to form chemical bonds. Helium is the most electronegative element followed by Fluorine.

Here in the given options, Oxygen is the most electronegative element. Fluorine is followed by Oxygen, Chlorine and Nitrogen is the most electronegative elements list.

Skimming is the removal of any material or particles that are floating on the surface. Hence, Removal of oil and grease from sewage - waste water is called as skimming.