The correct option is D Ashwamedha Yajna
The later Vedic period saw the emergence of new kingdoms across north India. Among those, Kuru, Panchal, Kosala, Magadha, Kashi, Anga were noteworthy. These empires fought each other to expand their territories and power. The kings tried to assert their power through various means. The Ashwamedha Yajna was performed to bring other rulers under subjugation. After successfully performing such Yajnas, rulers also took titles like samrat, meaning emperor, to flaunt his power and grandeur. Rajanyas or the smaller rulers who accepted his suzerainty attended such ceremonies. The empowered rulers became the centre of power during the later Vedic period.