SAT语法 In 1961 Rita Moreno gained fame and won an Academy Awa

SAT语法
In 1961 Rita Moreno gained fame and won an Academy Award （for her portraying ）Anita in the film adaptation of the groundbreaking Broadway musical West Side Story.No error
括弧里的为什么错了呀?
for 在这里面是个什么成分啊？引导原因从句吗？for she was portraying 是原句的完整句吗？这种题怎么能看出来是错的呀？我看着还觉得大家平时都有这么用的
错的部分呢，就是括弧里面的for her portraying