Option 1: Ashoka was a Mauryan ruler who lived before the Kushana dynasty and is known for his patronage of Buddhism. Therefore, Ashoka is not the correct answer.

Option 2: Vikramaditya was a legendary king who ruled over the Gupta Empire, not the Kushana dynasty. Therefore, Vikramaditya is not the correct answer.

Option 3: Kanishka is the correct answer. He was a Kushana ruler who ruled from around 127 to 150 CE. Kanishka is known for his patronage of Buddhism and his role in spreading Mahayana Buddhism throughout Central Asia.

Option 4: Kautilya, also known as Chanakya, was a scholar and advisor to Chandragupta Maurya, the founder of the Maurya Empire. He was not a ruler of the Kushana dynasty. Therefore, Kautilya is not the correct answer.

In conclusion, the correct answer is Option 3: Kanishka.