Which of these Roman gods doesn't have a counterpart in Greek mythology?

Janus is the correct answer as the Roman god that doesn't have a direct counterpart in Greek mythology. Janus is the god of transitions, beginnings, and endings, often depicted with two faces looking in opposite directions. His unique role as the god of doorways and the beginning and end of all endeavors has no direct equivalent in Greek mythology. While there are similar gods in Greek mythology, such as Hermes, who is associated with transitions, Janus stands alone in his distinct attributes and symbolism.