Final answer:
The correct answer is potassium (K), which is a highly reactive, low-density alkali metal that is an important component of fertilizers.
Step-by-step explanation:
The question revolves around identifying a highly reactive metal of low density that is also a key element in fertilizers. Among the options provided, potassium (K) fits this description accurately. Potassium is part of the Group 1 elements known as the alkali metals, which also includes lithium, sodium, rubidium, cesium, and francium. These alkali metals are characterized by their single valence electron, making them very reactive. Particularly, potassium is known for its role in biological systems and is one of the most abundant elements in Earth's crust. It is crucial for plant growth, which is why it is one of the three main elements found in fertilizer, alongside nitrogen and phosphorus.