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  

It is physical weathering

Explanation:

Physical weathering is a term used in science that refers to the geological process of rocks breaking apart without changing their chemical composition. Over time, movements of the earth and environment can break apart rock formations, causing physical weathering.