Do you know what happened to Mary yesterday? She looked like \_\_\_\_!

過去完成式。符合時態一致原則，且「look like / as if」後接子句表達誇張或與事實相反的假設語氣時，若主句為過去式，子句須用過去完成式（had + p.p.）。「look like she had seen a ghost」為形容極度驚恐的常見慣用語，為正確答案。