The correct answer is Option C,K, which stands for potassium. Potassium is an alkali metal and is highly reactive, especially with oxygen in the air. It reacts rapidly with atmospheric oxygen to form potassium oxide and other compounds, which causes the metal to tarnish or lose its luster quickly. This reaction can be so rapid that potassium must be stored under oil or in an inert atmosphere to prevent it from tarnishing or becoming a fire hazard. In contrast, some of the other elements mentioned have a slower reaction with air: Option A,Ba (Barium), is an alkaline earth metal and does oxidize in air, but not as rapidly as Option B, Be (Beryllium), is a relatively inert metal and does not tarnish quickly. It forms an oxide layer that protects it from further corrosion. Option D, Mg (Magnesium), is another alkaline earth metal that is more reactive than beryllium but forms a protective oxide layer which somewhat slows down further oxidation. The reactivity of these metals with air increases from beryllium (least reactive), to magnesium, to barium, and then to potassium (most reactive).