Which of the following is an indirect source? A. A source with no known author B. A source with no title C. A source that's cited in another source D. A source with no publication date

An indirect source is C. a source that's cited in another source.

Indirect means that it is coming from another source - it is not a direct one, where you'd just find a source (let's say in encyclopedia), and then incorporate it into your own work. Indirect source means that you take the sources that you found somewhere else (for example, Wikipedia's sources), and then incorporate those ones into your research.