Apiece of sodium metal can be described as
a pure substance and a compound.
an impure substance that is reactive.
an impure substance that is unreactive.
a pure substance and an element.

a pure substance and an element.

Explanation: Element is a pure substance which is composed of atoms of similar elements.

Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.

Mixtures are heterogeneous materials and are impure substances as they do not have uniform composition and the components are not evenly distributed throughout the material.  

Electronic configuration of sodium:

Sodium atom will loose one electron to gain noble gas configuration and form sodium cation with +1 charge.

Thus a piece of sodium is a pure substance and an element. Sodium is very reactive as it can easily lose its valence electron to attain stable configuration.