Men imagine that they communicate their virtue or vice only by _________ actions, and don't see that virtue or vice emit a breath every moment.

The second part of the sentence - “and don't………….. moment” is contrary to the first part as indicated by the “don't”. “Emit a breath” implies that men do good deeds from within; it also requires your inner force. As this force is hidden, they don't seem to realize it. Therefore, the opposite of hidden, not apparent characterize the blank. “Overt” is not hidden; open; apparent; hence (4) is the answer.