The statement best completes this diagram is Radical Republicans threaten to remove Andrew Johnson from office. Thus the option (A) is correct.
Who were Radical Republicans?
Radical republicans was the group of the politicians formed at the time of the civil war and continued till the period of reconstruction. They were working for the rights of the blacks.
Radical Republicans threaten to remove Andrew Johnson from office because he made the decision against the blacks and followed the policy of segregation of the blacks and whites.
Thus the option (A) is correct.
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