The correct answer would be "C", the Unites States did nothing at all.
After the fall of Phuoc Long, the US, a South Viatnamese ally, promised to retaliate if the North attacked the South. However, the North attacked, deciding to test the US resolve, due to their desire so secure transportation routes into the south. The US Congress, tired of a long war, deserted their allies and didn't keep their promise.
