The doctor wrote a recommendation letter for Robbie．

　　题干大意：医生为Robbie写了一封推荐信。题目的关键词是"recommendation letter"，对应原文第十段最后两句话：Two weeks later, when I arrived home from work, Robbie handed me a letter. He was informed to call Dr. Plum for an interview.原文是说两周后，我下班回来，Robbie给我一封信，信上说让Robbie打电话给Plum医生并约定面试时间。文中并没有说是Plum医生为Robbie写了一封推荐信，也许是别人替Plum医生写的，故选C。