I second-guessed myself after it wasn't accepted. You'd think it would be, being a direct cognate with exactly the same definition and a fairly common word at that...
It's not wrong, but it's orthogonal to the question. The quiz does not ask the user to translate from English to Spanish, but from Spanish to English. The question is correct. We also accept things like "guy" and "dude" for muchacho.
They way my favorite Spanish teacher taught it to me was that muchacho meant lad, as its meaning is closer to young man than boy and is used in a similar context to lad. However, boy is a perfectly reasonable translation.
Same, never studied spanish (except a few TV series with subtitles). Notably got "marinero" that's the least guessed one, similar to french "marin" (sailor)
Took a few year Spanish during schooling more than 15 years ago, didn't do anything with it since.....still got 19 out of 24 with it, and the other 5 really gave me a 'ah yes, that was it!' kind of vibe. Positively surprised by all of this!
Dad is a bit mor informal, like, normally you'd call your dad, well, dad, not father. In Spanish, Dad goes translated to papá
This is a synonym, also used in English:
https://www.merriam-webster.com/dictionary/seaman