Question 4
What holds a sodium ion and a chloride ion together in sodium chloride?
The right answer is the one marked above.
Electrostatic attraction between opposite charges. Once the electron has transferred, one ion is positive and the other negative, and that attraction is the ionic bond.
Sharing a pair of electrons would be a covalent bond, not an ionic one. Protons never move in a chemical reaction. Gravitational attraction between two ions is far too weak to matter at this scale.