Answer: Oxygen is an element.
Step-by-step 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.