Mr. X has chosen a specific flight route: Mumbai to Frankfurt (non-stop), Frankfurt to Sao Paulo (non-stop), and Sao Paulo to Buenos Aires (non-stop). To determine which sea Mr. X will likely fly over, we need to analyze the flight route.

Option 1: Tasman Sea - This sea is located between Australia and New Zealand, and considering Mr. X is traveling from Mumbai to Frankfurt, it is unlikely that he will fly over the Tasman Sea because it is not along the route.

Option 2: Labrador Sea - This sea is located off the eastern coast of Canada. Mr. X`s chosen flight route does not include any indication of flying over North America, so it is unlikely that he will fly over the Labrador Sea.

Option 3: Beaufort Sea - This sea is located in the Arctic Ocean, north of Alaska. As Mr. X`s chosen flight route does not include any indication of flying over the Arctic region, it is unlikely that he will fly over the Beaufort Sea.

Option 4: Black Sea - This sea is located between Eastern Europe and Western Asia. Since Frankfurt is located in Germany, Mr. X`s chosen route from Frankfurt to Sao Paulo indicates that he will likely fly over the Black Sea