Which of the following occurred after Allied forces invaded the Italian mainland in September 1943?

German forces retreated from Italy.

Axis forces repelled the Allies.

Italian forces quickly surrendered.

Allied forces declared victory in Europe.

Italian forces quickly surrendered, and the king ordered all men to stand down, which resulted in the Germans invading Italy and setting up a client state run by the now wanted Italian war criminal Mussolini

I believe the answer is: Italian forces quickly surrendered.

And Hitler's forces do not take italy's surrender very well. Not long after they heard the news, Hitler was furious and started to order several attacks to Italian armies within his vicinity. And this jeopardize the axis' master plan to win the world war II.