The correct answer is A) The unjust effects of the caste system.
The other options of the question were B) His wife's refusal to pay him a dowry. C) His father's defeat in a major battle with the Hindu army. D) His belief that the Untouchables could never be reincarnated into a higher caste.
The statement that can be identified as a cause of Siddhartha Gautama's dissatisfaction with Hinduism is the unjust effects of the caste system.
We are talking about Budha, whose original name is Siddhartha Gautama (563 BCE-483 BCE). He was considered the founder of the religious system called Buddhism. Buddha taught the Four Noble truths that were dukkha, the suffering; samudaya, the desire; nirodha, the ending, and magga, the correct path. And yes, Siddhartha Gautama's dissatisfaction with Hinduism was the unjust effects of the caste system.