According to which Buddhist text, Asoka distributed portions of the Buddha’s relics to every important town and ordered the construction of stupas over them?

The correct option is Ashokavadana.

Explanation:
The Ashokavadana (Narrative of Ashoka) is a 2nd-century CE Sanskrit Buddhist text that describes the birth, life, and reign of the Mauryan Emperor Ashoka the Great.
According to the legend detailed in this text, Ashoka collected the relics of the Buddha from the original seven stupas (after the eighth relic stupa at Ramagrama was guarded by the Nagas and left undisturbed). He then distributed these sacred relics among 84,000 urns and ordered the construction of 84,000 stupas (relic shrines) in every important town across his vast empire to promote Buddhism.

Why other options are incorrect:

• Sutta Pitaka and Vinaya Pitaka are parts of the early Pali Canon (Tripitaka) which primarily record the discourses of the Buddha and the monastic rules, respectively.
• The Mahaparinibbana Sutta is a Pali scripture that recounts the final days, death (Parinirvana), and initial distribution of the Buddha's relics immediately after his cremation among eight clans, which pre-dates Emperor Ashoka's reign.