Final answer:
The best completion for the diagram after the Aryan arrival in the Indus River valley is that Indians began to worship Brahman, as this reflects the religious and social changes brought by the Aryans.
Step-by-step explanation:
The cause being discussed is the arrival of Aryan people in the Indus River valley. Considering the historical context and the changes that came afterward, the statement that best completes the diagram would be the one related to the transformation in religious practice and social organization prompted by the Aryans. Consequently, Option 1: Indians start to worship Brahman is the most suitable completion for the diagram's effect, as Brahman is a concept in Hinduism which early Vedic religion evolved into. The Aryan migration led to the establishment of a new social structure and brought Vedic religion, which laid down the foundation for Hinduism, and Brahman eventually became a key part of this religious tradition.
The other options offered do not correctly represent the historical effects of the Aryan arrival. The social caste system was not abolished but rather reinforced by Aryan social structure, India did not lose the vast majority of its population immediately upon Aryan arrival, and Indians did not stop using a written language—Sanskrit, for instance, is notable for being an important literary and liturgical language of ancient India.