Final answer:
A spectrophotometer primarily measures the absorbance of light by a sample, which can identify which wavelengths of light an organism can absorb and can indirectly measure cell density.
Step-by-step explanation:
A spectrophotometer is an instrument used by scientists and researchers to measure and analyze the properties of light that is absorbed, transmitted, or emitted by a material. The statement that best describes what a spectrophotometer does is: "Measures the absorbance of light by a sample". By determining how much light at specific wavelengths is absorbed by a sample, such as pigments extracted from leaves, researchers can learn about the substance's characteristics and concentration. In the context of photosynthetic organisms, absorption spectra are generated to identify which wavelengths of light are absorbed by different pigments. This is crucial for understanding how these organisms utilize light for photosynthesis. Additionally, spectrophotometers can also measure the transmitted light through a sample and compute its turbidity or optical density, which can be used as an indirect measure of, for example, bacterial cell density.