The correct answer is B. methyl alcohol.
Ethyl alcohol is commonly made unfit for drinking by the addition of methyl alcohol, also known as wood alcohol. Methyl alcohol is a poisonous substance that can cause blindness and death if ingested. It is often used as a denaturant, which is a substance that is added to alcohol to make it unfit for drinking.
Potassium cyanide is a highly toxic substance that can cause death if ingested. It is not commonly used as a denaturant for alcohol.
Chloroform is a colorless, flammable liquid that is used as an anesthetic. It is not commonly used as a denaturant for alcohol.
Potassium chloride is a salt that is used in a variety of applications, including food additives, fertilizers, and pharmaceuticals. It is not commonly used as a denaturant for alcohol.