If you want to learn American English, I strongly suggest that you should watch films which                                   (以……为背景) the USA.

考查动词短语和时态。句意：如果你想学习美式英语，我强烈建议你应该观看以美国为背景的电影。空处位于which引导的定语从句中，作谓语动词，根据句意及汉语提示可知，应用动词短语be set in “以……为背景”，此处描述客观事实，用一般现在时，且在定语从句中，主语为关系词which，指代先行词films，故be动词用are。故填are set in。