It was the sixth trial, therefore you knew that Nezuko was the mastermind instead of Tsumugi

Nezuko: Ahahaha, took you long enough to find out that Team Danganronpa and Demon Slayer Corps worked together to form this killing game, My plan failed, I’m ashamed Miu: Rip Nezuko Kamado, Horror Factory Screamer, she won’t be missed Horror Factory Screamer: … Tsumugi: Told you I wasn’t the mastermind Shuichi: Nezuko Kamado, Ultimate horror enthusiast, you are the mastermind Maki: …. Himiko: I-I can’t believe it, Nezuko of all people? Kirumi: She won’t be missed Kaede: We trusted you Monokuma: Let’s go, it’s punishment time!!! Nezuko: I’m not even ashamed, I hope you have the day you deserve, because I will be looking down on you all