Oxygen is composed of only one kind of atom and can’t be separated into simpler substances. what type of matter is it?

Oxygen is an element.

Explanation:

A solution is defined as the a homogeneous mixture of solute and solvent.

Element is defined as the chemical specie which contains same type of atoms. For Example: Nitrogen (N), magnesium (Mg) etc...

A mixture is defined as the substance which is made by mixing two or more substances in any ratio.

Compound is defined as a chemical specie which is made by the combination of different atoms. For Example:   etc..

Colloid is defined as the mixtures where physical boundary is seen between the dispersed phase and dispersed medium.

We are given:

A chemical specie, oxygen which is made up of only one kind of atoms.

Hence, oxygen is an element.

indicate valleys, hills, steepness and gentleness of slopes.

explanation:

contour lines on a map indicate elevation, or depth. numeric notations on contour lines refer to the level of elevation, or height about sea level.

that's what i got you can count the number of atoms