Which of the following substances is a surfactant?

Sodium Stearate is the most common composition for surfactants which are Surface Active Agents, hence the word SurfActAnt. More than 50% of all surfactant deployment is with detergents, which we all use. A very interesting property for this compound is that it doesn't interact with other chemicals, and that permits it to be routinely approved by all regulatory authorities.

Surfactants are compounds that lower the surface tension between two liquids or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

This means that surfactants are a valuable addition to compounds which not only require surface dispersion but also, need government approval. Surfactant use in this context simplifies the approval process. Pesticides and herbicides are common examples which benefit from this effect.

For the more technically minded, surfactants form micelles which encapsulate water molecules, that in turn, allows the fundamental agent to do its work. In this regard, the surfactant acts as a carrier for the affiliated chemical agent. For instance, soap needs to penetrate the water to properly cleanse. Sodium stearate traps the water, the soap gets the dirt, and one simply rinses everything away.