Russia and Romania don't share a border. Ukraine borders both countries, the only country to do so. Romania is to the southwest of Russia.
The only country bordering Moldova (apart form Ukraine) is Romania. However, no country would satisfy the directions of your question.
The Black Sea is not a country but it is a sea, that can only be entered by ocean going ships from the Mediterranean Sea through the Bosporus. The Black Sea is surrounded by the coastlines of 6 nations. They are Turkey, Bulgaria, Romania, the Ukraine, Russia and Georgia.
Romania had interest only for Moldova, so it was not worried about Bulgaria and Ukraine
Russia, Turkey, Georgia, Bulgaria, Romania, Ukraine
Ukraine borders Russia, Belarus, Moldova, Romania, Slovakia and Hungary.
Ukraine is a country itself. If you mean "Is Ukraine only in one CONTINENT?", then yes, Ukraine is only in one continent called EUROPE.
Only Romania, of course.
Mexico is the only country that borders Texas.
SUDAN is the only country that borders both Egypt and Libya.
Portugal is a European country that borders only one other country, Spain.
Today Romania is a small country: only an area of 238 391 km2.