Anonymous

None of those is a sentence. They are all two sentences. One sentence would be "Emma knew what had happened to her son because Mia told her." That also seems more efficient than your examples.

Anonymous

For me its the first one but, what's to stop you from adding a few words at the end? NOTHING.

Anonymous

A. It is perfectly clear without adding any superfluous words.

Anonymous

i feel like its a

Anonymous

A. Is grammatically correct. Emma knew what had happened to her son. It was Mia who told her. It would, however, be more natural to say, "Emma knew what had happened to her son as Mia had told her."