Final answer:
An emulsifier is the correct choice for blending two substances together, such as oil and water, because it stabilizes emulsions by having hydrophilic and hydrophobic properties.
Step-by-step explanation:
The substance used to blend or mix two substances together is known as an emulsifier. An emulsifier is a compound that helps to stabilize emulsions, which are mixtures of two immiscible liquids like oil and water. Emulsifiers have a molecular structure that includes a hydrophilic (water-attracting) 'head' and a hydrophobic (water-repelling) 'tail'. The hydrophilic head is attracted to water, and the hydrophobic tail to oil, allowing the two to mix. Examples of emulsifiers include soaps and some detergents. Soaps are typically salts of long-chain carboxylic acids, such as sodium stearate, and detergents may include compounds like sodium dodecyl sulfate which are more soluble in hard water. Therefore, the answer to the question is D) Emulsifier.