Answer and Explanation:
Since this question has no options, I will offer a general explanation of the foreign expression.
The sentence we are analyzing here presents the French expression "carte blanche". Giving someone carte blanche means giving them total freedom to do something. For example: Mr. Johnson gave us carte blanche to choose the theme for our final paper.
In the passage, we are told "eight soldier-artists were given carte blanche". That means they were free to do whatever they chose to; they were free to use "any style or medium" they wanted.