Answer: b. mere gossip.
Thoreau argues that when life is not "inward and private" (when we do not concentrate on ourselves, as opposed to focusing on other people) conversation becomes superficial and degenerates in mere gossip. People end up talking about others, or about the affairs of the world. And we can be sure that people who only have this type of conversation have not had one with themselves in a long time, as they have nothing to talk about that does not involve others.
