The correct answer is $\boxed{\text{B}}$, ethene.
Ethene is an odd compound because it has an odd number of carbon atoms. The other options, propane, methane, and ethane, all have an even number of carbon atoms.
Ethene is a hydrocarbon with the chemical formula $\ce{C2H4}$. It is a colorless, flammable gas that is used in the production of plastics, rubber, and other chemicals. Ethene is also the simplest alkene, which is a type of hydrocarbon that contains one or more double bonds between carbon atoms.
Propane is a hydrocarbon with the chemical formula $\ce{C3H8}$. It is a colorless, flammable gas that is used as a fuel in heating and cooking appliances. Propane is also used as a refrigerant and in the production of plastics and other chemicals.
Methane is a hydrocarbon with the chemical formula $\ce{CH4}$. It is a colorless, odorless, and flammable gas that is the main component of natural gas. Methane is also used as a fuel in heating and cooking appliances.
Ethane is a hydrocarbon with the chemical formula $\ce{C2H6}$. It is a colorless, odorless, and flammable gas that is used as a fuel in heating and cooking appliances. Ethane is also used as a refrigerant and in the production of plastics and other chemicals.