Which of these contains the most authoritative data on diet and weight loss?

~ a university study
~ a low-calorie cookbook
~ a fitness training blog
~ a magazine survey

The correct answer here would be a university study. While the other mediums might have data that means well and is actually intended to help people lose weight, it's doubtful that this could be defined as authoritative. A university study, however, is likely to contain data obtained through scientific methods.