Question

What was the key challenge faced by the Allies during World War II?

Answer 1

I believe the answer is:  war was fought on multiple fronts

The multiple fronts make the Allies surrounded from both sides which limit their movement/mobilization

The alliance have to face the millitary threat from Germany on the enters front. On this front, The soviet union played  the most crucial role in preventing the nazi's advance.
From the western front, the western Europeans allies fought to stop the advance from japanese and italian army.

Answer 2

"War was fought on multiple fronts" is the one among the following choices given in the question that was the key challenge faced by the Allies during World War II. The correct option among all the options that are given in the question is the last option or the fourth option. I hope that the answer helped you.