Stupas gained significance in Buddhism because they housed what among the following options?

Sacred relics of Buddha

The correct answer is Option 3 - Sacred relics of Buddha

The tradition of erecting stupas may have been pre-Buddhist, but they came to be associated with Buddhism. Since they contained relics regarded as sacred, the entire stupa came to be venerated as an emblem of both the Buddha and Buddhism. According to a Buddhist text known as the Ashokavadana, Asoka distributed portions of the Buddha’s relics to every important town and ordered the construction of stupas over them. By the second century BCE a number of stupas, including those at Bharhut, Sanchi and Sarnath, had been built.