Answer: The substance that cannot be separated physically or chemically is an element.
Step-by-step explanation:
For the given options:
A solution is defined as the solution in which the components gets completely dissolved in it. Particles are evenly spread in these solutions. They can be separated by physical means.
An element is defined as the substance which is made of similar kind of atoms. All the atoms in an element have same atomic number. The atoms cannot be separated by any physical or chemical means.
Compound is defined as a chemical specie which is made by the combination of different atoms. For Example:
etc.. The atoms in a compound can be chemically separated.
A mixture is defined as the substance which is made by mixing two or more substances in any ratio. They can be separated by physical means.
Hence, the substance that cannot be separated physically or chemically is an element.