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  

the abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. the region occupied by the abdomen is termed the abdominal cavity. the stomach secretes acid and enzymes that digest food. ridges of muscle tissue called rugae line the stomach. the stomach muscles contract periodically, churning food to enhance digestion. the pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.

1 digestive system

2 stomach

3 bladder

4 carbon deoxide

5 tiger