The Shaka Samvat was started by

Concept

A calendar era is a system for numbering years from a fixed starting point, called its epoch. The Shaka Era has an epoch in 78 CE; in standard Indian historical and examination usage, its commencement is associated with the Kushan ruler Kanishka.

Application

Among the offered rulers, Kanishka belongs to the Kushan period associated with the beginning of the Shaka Era. Chandragupta Maurya and Ashoka belong to the earlier Mauryan age, while Vikramaditya is traditionally connected with a different era.

Contrast

• Chandragupta Maurya founded the Mauryan Empire in the late fourth century BCE.

• Vikramaditya is traditionally associated with the Vikrama Samvat, whose epoch is in the first century BCE.

• Ashoka was a Mauryan emperor of the third century BCE.

Cross-check

The Shaka epoch is 78 CE, whereas the Mauryan rulers in the options lived centuries earlier and the Vikrama tradition uses a different epoch. This chronological contrast supports Kanishka among the choices.

Therefore, the answer by value is Kanishka.