The correct answer is: B) The reviewer is speaking from an informed viewpoint.
The sources refer to the place the author gets the information he is giving us. They are divided in primary, secondary and tertiary sources For example, if the writer gives a statistic about the number of deaths caused by cancer his source could be the World Health Organization. In this case by providing sources to the review, the author can show the reader that the proper research has been done, they can also give a reference on where to look if they want more information about the topic.
In this case by giving the references the reader can be sure that the author didn't invent everything or give false information as it can be compared. The more sources he gives the more informed he appears to be.