(3) electronegativity is the correct answer.

This is the definition of electronegativity.

Alkalinity is the ability of a solution to neutralize acids.  Atomic mass is the mass of an atom and activation energy is the energy required to begin a chemical reaction.

(4) metallic because the valence electrons are mobile is the correct answer.

A property of a metal is that it’s an excellent conductor of electricity.  Metals exhibit "metallic bonding", where there are "metal ions" surrounded by a sea of mobile electrons. These free electrons are what easily conduct electricity.

(3) Both covalent and ionic is the correct answer.

The salt itself (the magnesium and nitrate ions) are held together by ionic forces. But the nitrate anion is held together by covalent bonds.