The correct answer is A) main topic or area of concern.
The subject of a source can best be described as the main topic or area of concern.
When doing research it is very important that you understand the main topic that has to be investigated or the area of concern that must be addressed by the research. Otherwise, the research can be wrong or biased. So you have to be careful of the subject of a source because it defines the most important part of the content. From them, will be able to follow the research and get important data, references, and results.