Translation:He sees the big smoke going up to the sky.
"the big smoke" is the problem, I think. "The large amount of smoke" is the closest I can think of that sounds right.
I put "heavy" smoke - which seems the most reasonable translation of "grosse" - but that was also rejected
Duo emailed me that "large amount of smoke" has since been added as an acceptable translation. Do try the "my answer should be accepted" checkbox-- it really does work to improve the program, even though you won't get your hearts back :)
I do agree that you need to add the word "cloud" making smoke into an adjective, or add some other qualifier to the word smoke to make this into good english.
Probably more idiomatic is - He sees the large cloud of smoke rising into the sky.
another thing is that in english you often hear 'he sees/saw the/a big/huge cloud of smoke going up/going up to the sky. you usually add a qualifier like 'cloud' or 'puff' to the word 'smoke' in this context just like judithscott pointed out :)
Why not use "epais" instead of "grosse" as the adjective in the French version? Then the phrase makes more sense in French and English.
Points off because I didn't correctly guess the nonsensical English answer! Ridiculous. (I'm pretty sure in a real-life translation scenario, my "he sees the large smoke plume rising to the sky" would be acceptable.) The big smoke. Sheesh.