Two or more combined substances that do not react form a _________.

solution
mixture
substance
compound

If I combined salt and pepper, they form a _______?

The correct answer is "mixture."

To arrive at this answer, we need to have a basic understanding of the terms provided as options.

1. A solution is a homogeneous mixture where one substance (called the solute) is dissolved into another substance (called the solvent) to form a single phase. In a solution, the solute particles are evenly distributed and cannot be visually distinguished from the solvent particles. For example, salt dissolved in water forms a solution.
2. A mixture is a combination of two or more substances that are physically blended together, but they do not chemically react with each other. In a mixture, the components can be present in different proportions and may be visually distinguishable from each other. For example, a mixture of sand and salt can be separated by physical means as their individual particles remain separate.
3. A substance is a particular kind of matter that has a specific composition and distinct properties. Examples of substances include elements like oxygen, hydrogen, or compounds like water (H2O).
4. A compound is a substance composed of two or more different elements chemically bonded together. Compounds have different properties from their component elements. For example, water (H2O) is a compound formed by the chemical bonding of two hydrogen atoms and one oxygen atom.

Based on the definitions provided, the- two or more combined substances that do not react form a mixture.