Final answer:
The answer to the question is Potassium (B). It fits the criteria of being a low-density, highly reactive metal used in fertilizer with similar but more expensive compounds compared to sodium.
Step-by-step explanation:
The student's question asks for the identification of a highly reactive, low-density metal that is one of the three main elements found in fertilizer and whose compounds are similar to sodium but are typically more expensive. After reviewing the options provided and considering the properties of the Group 1 alkali metals, the correct answer is Potassium (B).
Potassium is indeed a highly reactive low-density metal that is used extensively in fertilizers. It is one of the alkali metals, which are shiny, conduct heat and electricity well, and are highly reactive. These properties make potassium compounds very similar to those of sodium; however, the compounds of potassium are typically more expensive than those of sodium. Moreover, potassium is one of the three essential nutrients (N-P-K: nitrogen, phosphorus, potassium) commonly found in fertilizers. Considering the options provided, sodium (Na) and magnesium (Mg) are not the answers because sodium is not more expensive than itself, and magnesium is in a different group (Group 2: The Alkaline Earth Metals) and is not as reactive as potassium.