My opponent has chosen scissors in the last three consecutive rounds where we chose the same move. There's a good chance they'll stick with scissors again, or they might anticipate I'll switch to paper and choose rock themselves. By choosing rock, I can either win against scissors or tie against another rock. Since I'm already leading 4-1, even a tie or loss in this round will still give me the overall victory, but I believe rock gives me the best chance to win this specific round based on the pattern of play.