The correct answer is C. Dark chocolate has many health benefits.
Step-by-step explanation:
One main idea is a statement that encloses others and therefore the main statement the supporting ideas explain and prove. In the case of the statements "Dark chocolate is rich in minerals", "Dark chocolate reduces blood pressure" and "Dark chocolate prevents heart disease" these all refer to positive effects of dark chocolate in health including the minerals it provides and that it can help to prevent certain negative health conditions. According to this, the main idea these supporting ideas explain is "Dark chocolate has many health benefits".