Final answer:
Calcium carbonate is a compound that requires both calcium ions and carbonate ions to form, with the formula CaCO3 indicating a 1:1 ratio of these ions, the correct option is 3).
Step-by-step explanation:
To form calcium carbonate, both calcium ions (Ca2+) and carbonate ions (CO32−) must be present in the filtrate. The formula for calcium carbonate is CaCO3, which indicates that the compound is made of one calcium ion and one carbonate ion.
Calcium carbonate is a compound, not an element, because it consists of these two different ions in a fixed ratio of 1:1. In nature, the dissociation of calcium carbonate into its ions is a crucial step for the formation of substances such as calcite, aragonite, stalactite, and stalagmite within caves.