Translation:These old Russian cars are common in Hungary.
You need to report it. The team members don't read these discussions regularly or thoroughly, so they might not see your comment here.
Maybe the meaning is, that there are still many old russian cars in Hungary?
If it's going to be 'frequent', it has to be the adverb version...frequently, which was marked wrong. Common is certainly a better choice for correct English.
"Frequently in Hungary" sounds like they come and go, but frequently. Not quite what was intended, I think.
And why do you give the right answer frequent. In English there is as in all languages of the world the difference between common and fequent.
"Frequent" refers to time. But this is talking about how many there are within a certain space, ie, Hungary. "Frequent" wouldn't be appropriate, unless you were saying "These old Russian cars are frequently seen in Hungary." But in the given sentence, "common" is the right word to use.