The correct answer is (c). Barack Obama warned China not to threaten small countries like Vietnam and Philippines on ocean related issues.
In 2016, Obama visited the Philippines and gave a speech in which he said that the United States would “stand with our allies and partners in the region against any actions that could threaten the freedom of navigation or the international order that has brought so much prosperity and stability to the Pacific.” This was seen as a direct warning to China, which has been increasingly assertive in its claims to territory in the South China Sea.
The other options are incorrect. Vladimir Putin is the president of Russia. David Cameron is the former prime minister of the United Kingdom. Angela Merkel is the chancellor of Germany.