Final answer:
A cool region on the photosphere of the Sun is called a sunspot, which is cooler and darker than the surrounding areas due to concentrated magnetic fields.
Step-by-step explanation:
A cool region on the photosphere of the Sun is known as a sunspot. Sunspots are areas where magnetic field concentrations are high, resulting in a lower temperature compared to the surrounding areas. They appear darker than the rest of the photosphere because they are cooler. The photosphere is the layer of the Sun we typically see when looking at images of the star. Above the photosphere is the chromosphere, and beyond that is the corona, which is the outer atmosphere of the Sun and is hotter than the layers below it. During a solar maximum, the corona is even more vivid and conspicuous, indicating a time of increased solar activity, including the presence of more sunspots and associated phenomena like solar flares and coronal mass ejections (CMEs).