Final answer:
The region described in the question is the corona, which is the correct answer based on the provided choices, though the actual term for the Sun's interior zone with completely ionized gas is the radiative zone.
Step-by-step explanation:
The region of the Sun's interior where the gas is completely ionized due to extremely high temperatures is the radiative zone. This is not one of the options provided in the question, indicating a slight error in the question itself. However, the question seems to be describing the corona, which is the hottest part of the solar atmosphere and extends from the transition region that occurs just above the chromosphere.
The corona is where the temperatures range from a few hundred thousand to a few million Kelvin, and because it's less dense, photons can travel through it with relative ease compared to the deeper layers of the Sun.
The answer to the student's question based on the provided selections would be c) Corona, even though there seems to be some confusion in coupling the answer choices with the description in the question.