The metal _____ is always kept in kerosene?

The correct answer is Sodium.

CONCEPT:

• Alkali metals are highly reactive due to their large size and low ionization energy.
• Sodium is an alkali metal, it is always kept in kerosene to avoid contact with air and water.
• It burns vigorously with air or oxygen forming sodium peroxide.

                                  2Na + O₂  →  Na₂O₂

• It reacts with carbon dioxide forming a layer of carbonate on its surface.
• It also reacts with moisture and releases hydrogen.

                                  2Na + 2H₂O → 2NaOH + H₂

• All these reactions are exothermic and may catch fire.
•  Therefore Sodium cannot be stored in water.

EXPLANATION:

• Calcium, magnesium, and aluminium also react with oxygen and water, but these reactions are not as violent as that of sodium and other alkali metals.
• Calcium
• Reaction with water - Calcium reacts with water and releases hydrogen, this is a vigorous reaction, but not violent as that of sodium. 

Ca + 2H₂O → Ca(OH)₂ + H₂

•  Calcium gets slowly oxidised on exposure to air: 2Ca+ O₂ → 2CaO
• Magnesium​
• It reacts only with hot water.
• It burns in the air: 2Mg + O₂ → 2MgO
• Aluminium
• Aluminium doesn't react with pure water.
• Aluminium gets slowly oxidised on exposure to air:  Al + O₂ → Al₂O₃