The Gandhara School of Art flourished during the reign of Kanishka. The Gandhara School of Art was a distinctive blend of Greek, Persian, and Indian artistic styles and techniques. It emerged in the ancient region of Gandhara, which encompassed parts of present-day Pakistan and Afghanistan.

Option 1: Harsha - Incorrect. Harsha was an Indian emperor who ruled during the 7th century CE, which was after the decline of the Gandhara School of Art.

Option 2: Ashoka - Incorrect. Ashoka was an ancient Indian emperor who ruled during the 3rd century BCE, which was before the emergence of the Gandhara School of Art.

Option 4: Chandragupta II - Incorrect. Chandragupta II was an Indian emperor who ruled during the 4th century CE, which again was after the decline of the Gandhara School of Art.

Therefore, the correct answer is option 3: Kanishka.