which of these explains why the first stage of hinduism was known as brahmanism
The Brahmanism was the the very first phase of what is now known as Hindusim. It was a religion of priests/Brahmans who would perform Vedic sacrifice. So, they were the first Hindu priests to adopt the rituals from the Vedic. The post-Vedic Hinduism accepts the doctrine of Karma.
The Brahmas were the first Hindu priests who adopted rituals from the Vedic texts and contributed new ideas about spirituality to Hinduism.