What does densitometry measure?

Densitometry specifically measures the optical density of materials, commonly in the context of light-sensitive substances. This technique is often utilized in various fields, including biology and chemistry, to quantify the amount of a substance based on how much light is absorbed or scattered by it. For instance, in the laboratory, densitometry can be used to analyze gels for nucleic acids or proteins, helping scientists determine the concentration of these biomolecules based on the density of bands observed on the gel after electrophoresis.

While the other options involve analytical techniques related to different properties—such as concentration (which could use other methods like spectrophotometry), sizes of DNA fragments (which would be measured through techniques like gel electrophoresis), and osmolality (which measures the concentration of solute particles in a solution)—they do not pertain to the concept of densitometry, which is distinctly focused on optical density. This makes B the correct choice, as it accurately describes the core measurement that densitometry provides.