Answer: The right answer is "she is a fen Of stagnant waters: altar, sword, and pen, Fireside [...]."
Explanation: These are all references to various institutions and disciplines: "altar" stands for religion, "sword" represents the military, and "pen" represents literature. This is an example of metonymy because the writer is replacing those names (religion, literature) with names of things that are closely related to them (altar, pen). By doing so, his text is more concise, more creative, more symbolic and, therefore, more engaging and intellectually deep. In this poem, Wordsworth is invoking fellow poet Milton, who lived in the 17th century, since their country desperately needs him.