Scholars typically rely on a combination of evidence to understand the lifestyles of ancient peoples, including the Indo-Aryans. In this case, the most relevant option for proving that Indo-Aryans raised cattle and depended on their milk but later turned to agriculture would be:
The Vedas provide a record of how the Indo-Aryans lived.
The Vedas, which are ancient texts composed in Sanskrit, contain hymns, rituals, and cultural practices that reflect the early lifestyle of the Indo-Aryans, including their pastoral and agricultural practices. Additionally, archaeological findings and studies of material culture also support these interpretations.