The correct term for a substance that is the same composition throughout and cannot be separated by physical means, but can be broken down into smaller parts, is pure substance.
Pure substances can be further classified into elements (which cannot be broken down into simpler substances) and compounds (which can be broken down into simpler substances).
A mixture consists of two or more substances that can be physically separated. A solution is a type of mixture where one substance (the solute) is dissolved in another (the solvent). An element is a pure substance that consists of only one type of atom.
Since you mentioned the ability to be broken down into smaller parts, a compound (a type of pure substance) would be the most accurate answer if we consider the ability to be broken down chemically. However, based on your provided options, pure substance is the best fit.