The answer is "They helped spread Buddhism".
Stupa is a term used for Buddhist memorial landmark which usually contain holy relics related with the Buddha or other revered people. As indicated by Buddhist legend, especially the Mahaparinirvana, the remains of the Buddha had been shared among eight nations following his demise. Ashoka tried to reclaim the relics and distribute them among 84,000 Stupas built for religious purposes which greatly helped Buddhism spread in India.
Someone might say that as they hold the relics of elders option "C" might be applicable but that does not properly fit the description and purpose of a "Stupa" which primarily were built to hold the relics of Buddha himself who brought "salvation" or "Nirvana" for people not only in India.