Emperor Ashoka (reigned in the 3rd century BCE) had his edicts on Dhamma carved onto rocks and polished sandstone pillars across the Mauryan Empire — the earliest large body of decipherable royal inscriptions in Indian history.

The overwhelming majority of these edicts were engraved in the Brahmi script, written from left to right, which was the most widely used script across ancient India and became the ancestor of most later Indian scripts (including Devanagari and the Dravidian scripts). Their decipherment by James Prinsep in 1837 unlocked the reading of much of early Indian epigraphy. Hence the correct answer is Brahmi.

• Kharosthi, written right to left, was indeed used — but only for a few edicts in the north-western frontier region (such as at Shahbazgarhi and Mansehra), so it was regional, not predominant.
• Devanagari developed many centuries after Ashoka and could not have been used for his inscriptions.
• The Gupta script is a later evolutionary stage of Brahmi from the Gupta period (around the 4th–6th centuries CE), long after the Mauryas.