A conclusion is defined as any decision which based on reasoning. The surveys are typically based on reasoning where a large population is surveyed over any subject and their answers are then judged and a conclusion is drawn.
Here, Darius found that out of 600 people, 450 people said that when they have more success, they feel more happy. So, this concludes that there is some link between the two. Success and happiness go hand in hand and are directly proportional to each other.
Hence, option D is the answer.