The correct answer is D. However
Step-by-step explanation:
In texts, transition words connect two ideas or elements and clarify the relationship between the two elements connected. Due to this, the transition used depends on the relationship of connection between the ideas. In the case of the paragraph presented, the idea "these people may not realize that this claim is based on non-science" contrasts the first idea "some people who disagree with the idea of climate change may refer it as a "non-science" considering the second sentence explains the absurdity of those who claim the climate change is "non-science". According to this, a contrast transition such as nevertheless, despite or however is the best in this case. Thus, from the options given the best transition is however.