What would you be doing as a critical reader of an op-ed piece if you were looking for any potential bias the writer may have and the level of knowledge or expertise the writer may have on the topic?

The full question with choices was:

Which of the following would you be looking for as a critical reader with an eye toward ethos in an op-ed piece?

A) The writer's use of emotion to attempt to persuade the reader
B) The argument the writer makes and the strength of that argument
C) Any potential bias the writer may have and the level of knowledge or expertise the writer may have on the topic
D) None of the above.

The answer would be: C) Any potential bias the writer may have and the level of knowledge or expertise the writer may have on the topic

Ethos... = Writer or persona. You are looking for bias toward the subject. That's the writer.