C. Serbian nationalist formed an alliance with the ottomans
The others are wrong:
A) is wrong, because: The US joined the war because of the torpedoing of Lusitania, and the Zimmermann telegram, not because of the Russian Revolution
B) is wrong, because: Germany cannot invade in two fronts. Germany invaded only from the west, and that was mostly in WW2.
D) is wrong, because: Russia technically did not 'surrender' to Germany. Instead, they signed a armistice, which stopped fighting between the two.
hope this helps
