Firstly, both letters A and B should be identified as absurds, since Hinduism is definitely not monotheistic (Note: monotheistic religion means that only one god is worshiped in that religion). And I believe nor is Buddhism, since although the only 'worshiped' entity in it is Buddha, he is not considered to be a god, but a real man (a prince called Siddhartha Gotama who was 'enlightened' in life, and passed on the teachings of this enlightenment until his death).
And letter B is also absurd because the Buddha has no connection with the foundation of Hinduism. It already existed long before he was born (Evidence of its practice date before 4000 BC, being until nowadays the oldest fundamental religion still practiced on a large scale. Buddhism was found in the 6th BC century).
Letter C also turns out incorrect although Buddhism has some similarities with the Jainism (e.g.: in both of them, the idea of a god as a central figure is rejected, or at least not crucial).
Note: Jainism is one of the oldest religions in India, along with Hinduism and Buddhism themselves, created in the 6th century BC and based on the 'ainsa' philosophy, which means 'rejection to violence', in contradiction to hindu traditionalisms.
So saying that Hinduism is based on Jainism's beliefs is incoherent.
I did not find a source that states precisely that Buddha was in fact Hindu. It is said that after his realization that wealth and luxury couldn't bring him happiness, he went on to study many religions and philosophies for 6 years until he was finally enlightened at the age of 35 (and the resulting foundation of Buddhism). So by elimination, the letter D fits as the correct answer.
