Reported Speech

Dear all,

Would you please help me with this question about changing 'Direct Speech' into 'Reported Speech'? The following

question is taken from an exam paper.

"I saw my best friend yesterday', he said.

The options are:

A. He said he had seen his best friend the day before.

B. He said he saw his best friend yesterday.

C. He said he saw his best friend the day before.

I have opted both A & B because I have read about Reported Speech in some grammar books like Grammar in Use

(Raymond Murphy) or some other books that "Past simple" can be kept without changing especially in spoken English or

there are no confusion of time if we keep the past simple in indirect speech.

My concern is that:

A is the best choice.

B is possible providing that the time when the statement reported unchanged

(i.e. the author (he) and the speaker spoke the same content in the same day: yesterday)

C. Is C reliable? Can we still keep the simple past but change the adverb of time: change "yesterday" into "the day

before/ the previous day".

I am looking forward to receiving your comments.

Thanks in advance.