Final answer:
Sodium and chlorine are the most reactive pair, being an alkali metal and a halogen respectively, they react vigorously to form sodium chloride.
Step-by-step explanation:
The pair of elements that would be most reactive with one another is A) Sodium and chlorine. Sodium is an alkali metal that is highly reactive, particularly with halogens like chlorine. It is so reactive that sodium must be stored under oil to prevent reactions with air or water. Chlorine is a pale yellow, noxious gas and is highly reactive as well. When these two elements react, they form the ionic compound sodium chloride, which is essential for life, markedly different from the properties of both reactive elements.