The correct answer is option 1: Rig-veda. The Rig-veda is the oldest and most important of the four Vedas, which are the ancient sacred texts of Hinduism. It consists of a collection of hymns and verses that were composed by different seers during the Early Vedic Age, which lasted from approximately 1500 to 1000 BCE. These hymns provide invaluable information about the social, cultural, and religious aspects of the Early Vedic Age, including the lifestyle, beliefs, rituals, and deities worshipped by the ancient Vedic people. The Rig-veda offers insights into the early civilization and gives scholars a glimpse into the religious practices and worldview of the Vedic society. Therefore, studying the Rig-veda helps us understand the civilization of the Early Vedic Age.