which element is the most reactive?\no sodium\no nickel\no carbon\no oxygen

Brief Explanations:

Sodium is an alkali - metal. Alkali metals are highly reactive due to having one valence electron which they readily lose to achieve a stable electron configuration. Nickel is a transition metal and relatively less reactive. Carbon is a non - metal with moderate reactivity. Oxygen is reactive but not as reactive as sodium among these options in many common chemical reactions.

Answer:

sodium