Final answer:
Guru Nanak founded Sikhism primarily in opposition to the caste system prevalent in India during his time. Sikhism preached equality among all human beings and direct devotion to God, challenging societal and religious divides.
Step-by-step explanation:
The major reason for Guru Nanak's founding of Sikhism was his A) Opposition to the caste system. Guru Nanak Dev Ji was profoundly influenced by the cultural and religious milieu of his time which was marked by a rigid caste system. Sikhism emerged as a unique synthesis of Hindu and Islamic ideas but was revolutionary in rejecting the caste distinctions and emphasizing the equality of all human beings. This new monotheistic faith insisted on the importance of a direct and personal relationship with God, without the necessity of religious rituals or intermediaries such as priests.
Sikh teachings embody the idea of universal brotherhood and social equality, standing against the prevalent societal norms that divided people based on their birth into different castes. Guru Nanak Dev Ji advocated for a society where all individuals had the potential to connect with God, irrespective of their social status. This was a radical departure from the hierarchical structure sanctioned by the prevailing religious orthodoxy.