Final answer:
Sodium is known for having a very prominent and distinctive single absorption line in the yellow region of the spectrum, known as the sodium doublet, making it the element most associated with a single strong line in its absorption spectrum.
Step-by-step explanation:
The question pertains to the absorption spectrum of elements, which is a concept in physics and astronomy related to the identification of chemical elements in a star or the Sun based on their unique spectral lines. When observing the spectra of stars, each element emits or absorbs light at specific wavelengths characterized by these spectral lines. This forms the basis for spectroscopy, which is a tool for studying the elemental composition of astronomical objects.
In the case of a star with only one dark line in its absorption spectrum, we consider elements known to produce distinct single lines under certain conditions. Although several metals like magnesium, calcium, and iron can have multiple lines, sodium is known for having a very prominent and distinctive line, often referred to as the sodium doublet, in the yellow region of the spectrum despite having other lines under different conditions.
However, the question asks for an element with only one dark colored line, which is a more peculiar and hypothetical situation. In reality, elements tend to have multiple lines. Thus, without additional context, this question may not have a definitive answer in practical spectroscopy. But for the sake of answering the question based on typical stellar spectra and assuming the simplest case, sodium would be the best choice because of its prominent single absorption line in many circumstances.