Translation:The environment of his home is cozy.
My first thought was the environment around the house rather than in it. If in, then perhaps 'home,' is better than 'house.' Parallel with the question of inside or out, is the question of whether we are talking about the natural, physical environment principally outside the house or the psychological or emotional environment in the home. Did anyone try: His home environment is cosy. Still, whether inside or outside, none of the possible direct translations I can come up with sound very natural in English. A psychologist or social worker might speak about one's home environment, but would I be likely to say that of my neighbor? Hm.
I believe 环境 relates to the physical surroundings and tangible environment rather than the situation or circumstances of a family etc. That being said, I am not a chinese social worker.
As ever, when it comes to duolingo, we needn't concern ourselves with how usual or "natural" sounding the English version of the sentence is. Chinese and English are different languages and the things people say in them reflect different ways of seeing the world, that's part of the beauty of learning other languages. Our job is to translate as a means to learning so it doesn't matter how unlikely the English version of the sentence is so long as it is a grammatically correct translation of the original sentence that is as direct as possible.
The problem is that sometimes we just fail to understand that direct translation. I agree with you about "usual or natural" part, it doesn't even hurt me if the translation is not grammaticaly correct (as long as correct one is also accepted) - the one thing I want to understand is what does the chinese senence mean.
So these usual English versions just work as explanations. It would be perfect to have both somehow,
Thank you for your explanatios
My personal walking, talking dictionary says the sentence means "His house/home is cozy". If the Chinese sentence needs to be translated literally, I would suggest translating "huanjing" (lit. "environment") somewhat metaphorically into something like "atmosphere" or "feeling".
"cozy" is the standard American -- and apparently Canadian -- spelling; hence the spelling used by the majority of English speakers. Hence it should be used here. Also, "cozy" fits the pronunciation better; at a time when millions of English-speaking adults ar functionally illiterate, the relevant people should not be able to, in good conscience, choose the spelling "cosy" over "cozy". The basic premise, namely that the less-transparent the orthografy, the harder and more laborious it is for children to learn to read and write, and the more likely people ar to fail to learn it properly, is a well-documented fact, not an opinion.