"Take care of the environment."
Translation:Dbajcie o środowisko.
"Troszcz się o środowisko" and "Troszczcie się o środowisko" is correct and should also be accepted, although we rather use the verb "troszczyć się" in context of taking care of given living beings or members of family.
It is accusative, that has the same form as nominative for neutral nouns. "O" take locative when it means "about" and accusative when it means "against" (or in this case, "of")
I cannot find a resource to clarify it. All learners' resources do not include this kind of things, all Polish resources have accusative and locative mixed in the first "about" meaning.
So I'm creating rule while writing and can be completely wrong, but I think
in the subject (think, write, a book, talk) - instrumental
of (take care, worry) - accusative
or maybe it's easier to think of dbać o; troszczyć się o; martwić się o; as exceptions
"o" takes Locative ("o środowisku") when it means "about", which I guess statistically is most of the cases. But sometimes there are other meanings of "o", which need Accusative, and that's one of such situations.