The correct answer is A. ethyne.
Ethyne is a hydrocarbon with the chemical formula $C_2H_2$. It is also known as acetylene. Ethyne is a colorless, flammable gas that is used in welding and as a fuel.
Ethyne is the simplest alkyne, which is a hydrocarbon that contains a triple bond between two carbon atoms. The triple bond makes ethyne a very reactive molecule.
The acidic hydrogen in ethyne is the hydrogen atom that is attached to the carbon atom that is part of the triple bond. This hydrogen atom is acidic because it is easily removed as a proton, $H^+$.
The other options are incorrect because they do not contain an acidic hydrogen atom.
B. Ethene is a hydrocarbon with the chemical formula $C_2H_4$. It is also known as ethylene. Ethene is a colorless, flammable gas that is used in the production of plastics and other chemicals.
C. Benzene is a hydrocarbon with the chemical formula $C_6H_6$. It is a colorless, flammable liquid that is used in the production of gasoline and other chemicals.
D. Ethane is a hydrocarbon with the chemical formula $C_2H_6$. It is a colorless, flammable gas that is used as a fuel.