To solve this question, the user needs to know about the Sanchi stupa and its history.

The correct answer is:

C. Ashoka

Explanation:

The Sanchi stupa is a Buddhist monument located in the Indian state of Madhya Pradesh. It was constructed by the Mauryan emperor Ashoka in the 3rd century BCE. The stupa was built to house the relics of the Buddha and to serve as a center for Buddhist worship. The stupa at Sanchi is considered one of the oldest and most important Buddhist monuments in India. Therefore, option C is the correct answer.

Option A, Harsha, is incorrect. Harsha was a king who ruled India in the 7th century CE, long after the construction of the Sanchi stupa.

Option B, Ajay, is incorrect. There is no historical figure with the name Ajay who is associated with the construction of the Sanchi stupa.

Option D, Buddha, is incorrect. The Sanchi stupa was built to house the relics of the Buddha, but the Buddha himself did not construct the stupa.