"The green boy."
Translation:An buachaill glas.
Because the course creators didn’t anticipate it as a correct answer. There is a distinction between the shades of glas and uaine, but since both translate as “green” in English, and there’s no context to this sentence, both of them should be accepted. Be sure to report it as an error when opportunity allows so that uaine will become accepted.
As the idiomatic meaning stems from the unripe/immature aspect of green crops, it is used that way in Irish too.
From the FGB entry for glas:
(c) Immature, unseasoned. Slat ghlas, green rod. Adhmad glas, green timber. Ógánach glas, callow youth. Tá sé glas go fóill, he is young yet. (d) Raw, inexperienced. Lámh ghlas, prentice hand. Saighdiúirí glasa, raw troops. Bheith glas ar rud, to be unused to sth. Ba ghlas m’eolas air, little I knew of it. Nach mé atá glas agat? How gullible do you think I am?