The correct answer is: topic sentece
The topic sentence is the most important sentence (or senteces) of a pararaph as it reveals the main point of it. By reading it, the relation between the paragraph and the thesis of the essay is shown as well as it tells the reader what to expect from the paragraph itself. It is said that they argue more than report as they make a point and stress it's importance.
As we can see, the topic sentence is not a supporting detail as it makes a point by itself. It needs to be supported by descriptions, facts and illustrations.