What keeps an ionic bond held together?

a)gravity

b)the sharing of electrons

c)energy of valence electrons

d)attraction between oppositely-charged ions

D) Attraction between oppositely-charged ions.

In ionic bonds, one atom is negatively charged, and one atom is positively charged. The positively charged atom gives its electrons, and the negatively charged atom receives the electrons--thus, the opposite charge attracts them to one another and holds them together. That simple. 

Table salt has a uniform and definite composition. It is not a mixture because the sodium and the chlorine are chemically bonded, meaning you cannot take the sodium out of the chlorine or vice versa.

Explanation:

All samples of salt are chemically identical, whereas a mixture can vary. For example, salt water is a mixture because you can have differing amounts of salt or water. The composition is not the same all the way throughout.