The correct answer is the final one: Lucy was little known because she frequented "the untrodden ways / Beside the springs."
Rustic means "relating to the countryside; rural." The poem describes a young country girl. We know that she is from the country because Wordsworth describes a country scene -- he mentions a spring, a flower, a mossy stone. Also, the fact that she frequents the "untrodden ways" is another indication that she is not from a city. She "lived unknown," meaning she lived a simple, rustic life.
