Final answer:
Li is the most reactive metal among Na, Li, Mg, and Ca due to its belonging to the alkali metals, which are highly reactive.
Step-by-step explanation:
The most reactive metal among Na, Li, Mg, and Ca is Li. The alkali metals, which include Li, Na, K, and others, are the most reactive among the metals in the periodic table. They react vigorously with water and have similar chemical properties. Li, Na, and K are much more reactive than Ca.