The correct answer is (c), Kushinagar.
Buddha spent the last rainy season of his life in Kushinagar, a town in the Indian state of Uttar Pradesh. He died there shortly after, at the age of 80.
Shravasti was an important city in the Buddha’s time, and he is said to have spent many rainy seasons there. However, there is no evidence that he was there during his last rainy season.
Vashali was another important city in the Buddha’s time, and he is also said to have spent many rainy seasons there. However, there is no evidence that he was there during his last rainy season.
Sarnath is the place where the Buddha gave his first sermon after his enlightenment. It is a major Buddhist pilgrimage site, but there is no evidence that the Buddha was there during his last rainy season.