Study skills are not subject specific - they are generic and can be used when studying any area. You will, of course, need to understand the concepts, theories and ideas surrounding your specific subject area. To get the most out of your studies, however, you'll want to develop your study skills.
I believe it is C, an in-text citation because it isn't A since you've cited the source, it's not a full works cited like D, and it's not in quotes so it's not a direct quotation like B. Hopefully that helps you out!