Which of the following substances is NOT classified as an organic compound?

Organic compounds are primarily characterized by the presence of carbon atoms in their molecular structure, typically bonded to hydrogen and other elements. In this context, hydrochloric acid (HCl) stands out as the correct answer because it does not contain carbon, which is a fundamental requirement for a substance to be classified as organic.

Acetic acid (CH3COOH), glucose (C6H12O6), and methane (CH4) all contain carbon atoms along with hydrogen and, in the case of acetic acid and glucose, other elements such as oxygen. These compounds demonstrate typical properties of organic substances, including the ability to participate in various chemical reactions that are characteristic of organic chemistry.

In summary, hydrochloric acid is inorganic because it consists solely of hydrogen and chlorine without any carbon, distinguishing it from the organic compounds listed in the question.