By the beginning of the Christian era, the most preferred stone for construction of the stupas in the Swat valley was?

Correct option is D

By the beginning of the Christian era, the most preferred stone for building stupas in the Swat Valley, and the surrounding region of Gandhara, was grey or green schist. This preference extended for a significant period, particularly up to the third century CE. It was favored for its availability and suitability for construction and carvings.