Explanation:

- Option 1: Kadphises in 58 BC

Kadphises was a Kushan ruler, but he didn't start the Saka era and 58 BC is not associated with him.

- Option 2: Rudradaman I in 78 AD

Rudradaman I was a Saka ruler, but he did not initiate the era. 78 AD is the correct year, but not the right ruler.

- Option 3: Vikramaditya in 58 BC

Vikramaditya is linked to the Vikram Samvat era, which starts in 58 BC, but this is different from the Saka era.

- Option 4: Kanishka in 78 AD

Correct! The Saka era is said to have been started by Kanishka, the Kushan emperor, in 78 AD. This era is also called the Shaka era and is used in the Indian national calendar.