Final answer:
During a total solar eclipse, observers in the path of the Moon's darkest shadow witness a total eclipse, while those in a larger area see a partial solar eclipse. An annular eclipse occurs when the Moon is too far to cover the Sun completely, which is not the same as a total solar eclipse.
Step-by-step explanation:
Determining what must be visible during a total solar eclipse requires understanding the geometry of solar eclipses. When the Moon's darkest shadow, or umbra, intersects with Earth and the alignment between the Earth, Moon, and Sun is just right, observers in a small area on Earth experience a total solar eclipse.
This is a unique event where the Moon completely covers the Sun, and the sky darkens enough to view planets and brighter stars. At the same time, a larger area experiences a partial solar eclipse, where only part of the Sun is blocked by the Moon. Therefore, the correct answer is a. partial solar eclipse.
It is also important to note that an annular eclipse occurs when the Moon is too far away to fully cover the Sun, leaving a ring of light visible, which is different from a total solar eclipse. Total lunar eclipses and penumbral lunar eclipses, while interesting astronomical events, are not relevant to observing a solar eclipse.