Hello, Abdullah,
There isn't a really good answer in the options, but I suspect the sought answer is "sodium transfers an electron to chlorine." I say "not a good answer," since that is the outcome, not the reason, as I'll describe:
An electron leaves sodium and moves to chlorine. Unlike covalent bonds, the chlorine doesn't share the electron with sodium much at all. It becomes an ionic bond. The two elements are attracted to each other due to the electrostatic attraction of the two opposing charges (Na is a positive 1 and chlorine is a negative 1).
But the questions asks for the "reason." The reason is that both elements in a lower energy state when the lone electron in sodium's outer (s) shell moves to the chlorine's outer p shell, and gives that p shell a "full octet" of electrons.
Bob
