Which of the following best describes a macromolecule?

A macromolecule is best defined as a large organic molecule, such as proteins, nucleic acids, carbohydrates, or lipids. These molecules are typically composed of hundreds or thousands of atoms and play crucial roles in the structure and function of cells. For instance, proteins are made up of long chains of amino acids, and nucleic acids, like DNA and RNA, consist of long sequences of nucleotides.

In contrast, the other options fail to capture the essence of what constitutes a macromolecule. Small organic molecules, such as glucose, are not large enough to be categorized as macromolecules. A carbohydrate with only one sugar unit, known as a monosaccharide, does not meet the criterion of being a macromolecule due to its small size. Lastly, inorganic compounds like water are not considered organic molecules at all, thus they do not fit the definition of macromolecules.

Therefore, option B accurately encompasses the essential characteristics of macromolecules with regard to their size and organic nature.