Spanish: Yo soy El (masculine) más joven..
Yo soy LA ( female) más joven..
English I am the youngest girl/ boy in the family . I suppose
The youngest one - el o la más joven. ¿La camisa roja o la ( camisa) azul? The red shirt or the blue shirt.? ¿La de color rojo/ la roja o la de color azul / la azul? ¿Ésta o esa o aquella? - The red one or the blue one? This one or that one?
That is exactly what I did, and it would be normal to say that, more so than "the youngest one". It was the only one marked wrong in my test out too so it hurt more!
Why not el menor? Or la menor but you can't assume anything in Duo from the speaker's gender.
"I am the youngest of the family" A direct translation that works, but still got marked wrong. Come on Duo!!!!
Either it's fixed now (because that's what I put, and it was marked correct), or it was one of those things that someone at DL told me happens occasionally (theoretically not often), that someone is marked incorrect when they are correct.