Which of these nations borders the Black Sea? Poland, Moldova, Ukraine or Greece. Of these options only Ukraine borders the Black Sea.
Crimea is a peninsula in the Black Sea. Officially, it used to be part of Ukraine but Russia annexed it, meaning Russia took it over in 2014 after Crimeans voted to be part of Russia. At that time, Russian President Vladimir Putin said Crimea's government followed international law by holding the vote but critics say it was not a fair election.
The Obama Administration, which led the U.S. in 2014, said Russia's annexation of Crimea broke international law and that the U.S. wouldn't recognize it. Most other nations have also refused to recognize Crimea as being part of Russia but the peninsula has functioned as a Russian Republic since 2014 and it gives Russia a strategically important warm water port in the Black Sea.
What's happened in Crimea is one example of a deep divide in Ukraine, the country used to be part of the Soviet Union which was dominated by Russia.
Today, many Ukrainians identify themselves as Russian while many others do not and want to have closer ties with Europe.
There's been an armed conflict in Ukraine since 2014 with Russia supporting the side that wants independence from the Ukrainian government. Tensions have spilled from land to sea.