Electrons are important in chemical reactions. Since the elements in a family tend to behave similarly in chemical reactions, one may conclude that:

One may conclude that they have an identical number of electrons in their outermost shell, therefore the can be substituted for one another.

Explanation:

Elements in the same family tend to behave similarly in chemical reactions  because they have similar number of valence electrons in their outermost shell and they also have similar chemical properties.

Valence electrons are responsible for reactions between elements. The valence electron makes it possible for atoms to combine and react, therefore, elements in a family behave similarly in chemical reactions because they have an identical number of electrons in their outermost shell.

C. Six ammonia molecules are formed, with zero molecules remaining.