Stupas are primarily associated with option B: Buddhist commemorative burial mounds.
A stupa is a dome-shaped structure that serves as a Buddhist shrine or monument, typically housing relics associated with the Buddha or other revered figures. These structures were originally built as burial mounds containing the remains of the Buddha or other saints and are significant in Buddhist practice and meditation. They represent the enlightened mind of the Buddha and are a focal point for devotion and worship.
Options A, C, and D do not accurately describe what stupas are.