[tex] {\qquad\qquad\huge\underline{{\sf Answer}}} [/tex]
Sodium is a highly reactive alkali metal and it looses an electron to gain a positive charge on it and bromine is a halogen that is highly resctive as well and it gains a negative charge by doing so.
And hence they form ionic bond, and the compound is held together by the attraction between the two charged ions.
So, correct choice is : A. oppositely charged ion attract.