What is the only element with the symbol Na?

The element with the symbol Na is sodium. This symbol comes from the Latin name for sodium, "Natrium." Sodium is an alkali metal and is highly reactive, often found in nature in compound form, such as in table salt (sodium chloride). It plays a crucial role in various biological processes, including nerve impulse transmission and fluid balance in cells.

Calcium, magnesium, and potassium are distinct elements with their own unique symbols—Ca for calcium, Mg for magnesium, and K for potassium. Each of these elements has different properties and uses in biological and chemical contexts. Sodium's unique placement in the periodic table and its reactive nature set it apart from these other elements, affirming that B is indeed the correct answer.