Final answer:
Aluminum is the least easily oxidized metal among the options given as it is neither an alkali nor an alkaline earth metal and is less reactive according to the activity series.
Step-by-step explanation:
The question seeks to determine which metal is the least easily oxidized among sodium, aluminum, zinc, and calcium. According to the activity series of metals, the inert metals at the bottom of the series are the least likely to be oxidized. These include precious metals and coinage metals like platinum, gold, silver, and copper.
Considering the metals listed in the question and knowing that alkali metals and alkaline earth metals are at the top of the activity series and thus more reactive, we can infer that aluminum (Al), which is neither an alkali nor an alkaline earth metal, would be the least easily oxidized among the options provided.