To answer this question, we need to have knowledge of Hindu mythology.
Option A) Uttanka - This option is incorrect. Uttanka is a sage in Hindu mythology, but he is not associated with eating the demon Vatapi.
Option B) Vasistha - This option is incorrect. Vasistha is a sage in Hindu mythology, but he is not associated with eating the demon Vatapi.
Option C) Agastya - This option is correct. Agastya is a revered sage in Hindu mythology and is known for his encounter with the demon Vatapi. According to the legend, Vatapi had the power to take the form of a demon and would invite unsuspecting travelers to his home. Once the traveler arrived, Vatapi would transform into a delicious meal. Agastya, however, was able to outsmart Vatapi by digesting him immediately after eating, preventing Vatapi from regenerating.
Option D) Gautama - This option is incorrect. Gautama is a sage in Hindu mythology, but he is not associated with eating the demon Vatapi.
Therefore, the correct answer is Option C) Agastya. Agastya is the sage who ate the demon Vatapi according to Hindu mythology.