Final answer:
Oxygen is one of the most reactive elements. This reactivity, coupled with its abundance, makes the chemistry of oxygen very rich and well understood.
Step-by-step explanation:
Out of the elements provided - Sodium, Nickel, Carbon, and Oxygen, the most reactive element is Sodium. Sodium is one of the Alkali Metals, which are known for their high reactivity, particularly when they come into contact with water. Oxygen also shows high reactivity as it readily makes compounds with most other elements and is crucial for oxidation-reduction reactions.
Although not as reactive as fluorine, which is the most powerful oxidizing agent among all the elements, Sodium and Oxygen are both highly reactive in their respective categories (metal and non-metal). Nickel and Carbon exhibit reactivity as well but to a lesser extent compared to Sodium and Oxygen.