Which molecule is specifically characterized by having one carbon atom?

The molecule that is characterized by having one carbon atom is methane. Methane is the simplest alkane and has the chemical formula CH₄. In this molecule, one carbon atom is bonded to four hydrogen atoms, making it a fundamental example of a hydrocarbon. Methane is colorless, odorless, and is often used as a fuel source.

In contrast, ethane contains two carbon atoms, propane consists of three carbon atoms, and butane has four carbon atoms. Each of these molecules is larger and more complex than methane, demonstrating how the number of carbon atoms determines the basic structure and name of the alkane. Thus, methane, with its single carbon atom, uniquely fits the description provided in the question.