选择B.is regarded
“be regarded as ”的中文意思是“被认为是·······的”
在这个句子当中“who ”引导定语从句,“who”指代的就是“their son”,在从句中充当的成分为主语,从句缺少谓语动词,A选项regarded是非谓语,不能选,选择B则语句成分完整了.
tom's parents are proud of their son,who is regarded as the best doctor in town．
翻译：汤姆的父母以他们儿子为荣,他们的儿子被认为是小镇里最好的医生.