A. An element.
As long as it's on the periodic table, it's always an element.
A compound is made up of two or more elements in a chemical reaction, resulting in covenant and/or ionic bonds.
A homogenous mixture (a solution) is a combination of two or more elements physically without causing a reaction (like salt water or applejuice).
Heterogeneous mixtures are physical combinations that mix but don't appear uniform (think of cereal in milk.)