Which of these is not considered a pure substance?

a)
an atom

b)
a mixture

c)
an element

d)
a compound

A mixture

Explanation:

A mixture is made up of different elements/compounds hence it is not pure

Atoms are the fundamental building blocks. A single atom is so small that it is impossible to see it. It is a pure substance since it cannot be broken down further

Elements are pure substances for example hydrogen, oxygen, sodium copper iron etc.

A compound is also a pure substance since it is made up of two or more elements  

1. loamy sand is about 80% sand with 10% clay and defined mostly by sand particles with some clay to provide structure and fertility. the sand particles are easily visible and the soil holds structure but breaks apart easily.

2. scott sand can improve his soil quality by incorporating more silt and organic matter into the soil. peat moss and organic matter can also be incorporated.

most community's take hundreds of years to become stable