D. "this was an uneasy secret..."
The first one since it uses the Oxford comma.
C. Symbol
Subject is the focus of the literature/art, context is what it's about or the surrounding meaning, theme is the subject or topic of lit or a work of art (though it can also be a main idea).
Symbol is the most correct answer, because it is a <em>thing, </em>object or detail, that represents an idea, as said.
