1. A dacha is a Russian country house.
2. It is a Russian country house.
3. One is a Russian country house.
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Which answers are grammatical? Which one is ungrammatical?
All are grammatical -- grammatically correct -- but #3 doesn't make sense unless someone is looking at two or more photos, only one of which is a dacha.
All three options provided are grammatically correct. They each convey the same meaning, explaining that a dacha is a Russian country house. However, option 3 is less commonly used as it lacks an article ("a") before "dacha." It is more natural to include the article when describing or defining a singular object.
