The main reason northern states opposed slavery was largely D: it was an unfair business advantage.
While there were multiple factors that contributed to the opposition to slavery, including moral, social, and economic reasons, many in the North saw slavery as a system that provided an unfair economic advantage to slaveholding states in the South. They believed it distorted free labor markets and competition. Additionally, though the other options have some relevance, the economic perspective was a significant motivator for opposition in the northern states.