The correct answer is A. Upagupta.
Upagupta was a Buddhist monk who lived in the 3rd century BCE. He is said to have been the spiritual advisor to Emperor Ashoka, who converted to Buddhism after the Battle of Kalinga. Upagupta is also credited with spreading Buddhism throughout India.
Vasubandhu was a Buddhist philosopher who lived in the 4th century CE. He is considered one of the most important figures in the development of Mahayana Buddhism. Vasubandhu is best known for his work on the Abhidharma, a collection of texts that systematized Buddhist doctrine.
Ambhi was a king who ruled the kingdom of Avanti in the 3rd century BCE. He was a contemporary of Emperor Ashoka and is said to have been a patron of Buddhism.
Asvagosha was a Buddhist philosopher and poet who lived in the 2nd century CE. He is best known for his work on the Buddhacarita, an epic poem that tells the story of the Buddha’s life.
Upagupta is the most likely answer to the question because he is the only one of the four figures who is known to have been a spiritual advisor to Emperor Ashoka. Vasubandhu, Ambhi, and Asvagosha were all important figures in the development of Buddhism, but they are not known to have had any personal connection to Ashoka.