Option d is the correct answer. The only pillar on which Ashoka declares himself as the emperor of Magadha is the "Bhabru pillar." This declaration can be found in Minor Rock Edict No. 3, which is also known as the Bhabru Edict. The pillar is associated with Bairat, where it was discovered. In this edict, Ashoka expresses his reverence and faith in the Buddha, Dharma, and Samgha (Buddhist monastic community), and he mentions his role as the Magadha king.