Which of the following substances is known to be an asphyxiant?

The correct choice identifying a substance known to be an asphyxiant is acetylene, represented as C2H2. An asphyxiant is a gas or vapor that can displace oxygen in the air and thus lead to a deficiency of oxygen for breathing, which can be hazardous. Acetylene is flammable and can cause an oxygen-depleted environment, especially in confined spaces, making it a potential asphyxiant if it displaces breathable air.

The other substances listed, like ethane (C2H6), propane (C3H8), and butane (C4H10), while also flammable hydrocarbons, are not classified as asphyxiants in the same way. They can cause oxygen displacement, but acetylene is specifically noted for its ability to create hazardous conditions due to both its flammability and how it interacts in certain environments, thus solidifying its recognition as an asphyxiant.