The Final Selection is the last hurdle for aspiring swordsmen to join the Demon Slayer Corps. How did Zenitsu pass the final selection?
Zenitsu was able to pass the final selection by hiding. He told Tanjiro he “was lucky and survived,” meaning he likely avoided demons for the required 7 days to pass the selection and did not engage in combat.
Only 4 students in total survived the Final Selection. Tanjro, Zenitsu, Genya, and Kanao.
Tanjiro Learns How Zenitsu Was Able to Pass The Final Selection
After the final selection, Tanjiro went on a few low-rank demon slaying missions. A crow instructs him to move South East, and he comes in contact with Zenitsu begging a woman to marry him.
In chapter #20, Zenitsu cries to Tanjiro that he’ll pass away long before his next job is completed. Tanjiro questions how someone who self-proclaims to be a swordsman could act so cowardly.
I went through a hellish training regiment every day, and during that time I really thought I might as well just die! And I thought I was really going to die during the final selection! In the end, I was lucky and I survived. Then I ended up back in that hellish lifestyle again! Chapter 20
Zenitsu to Tanjiro / Chapter #20
Zenitsu’s claim of being “lucky” to survive the selection makes sense. It was never required of the young swordsmen to slay any particular amount of demons. The only requirement was to survive in the demon-infested woods for a total of 7 days.
It was likely that Zenitsu was able to pass the exam by hiding for 7 days. He doesn’t defeat his first demon until the Drum House arc, so it would make sense that he hid away for the duration of the final selection. Zenitsu likely went out during the day to change his position and gather supplies, slipping back away into obscurity during the night.