This could be, "I am not thin" also, there is no context here so it doesn't seem correct to only have one answer for something that could be either 'Eu sunt' sau 'Ei sunt' Ele sunt'. Or am I completely mistaken
Subțiri is plural. For I, it would be subțire
That would be Nu sunt subțire, if I'm not mistaken. Subțiri is plural.
Edit: oops, NickyVlad had already answered, for some reason the duolingo app didn't show any answers.
Adjective ia plural form