Why were many of the delegates to the constitutional conventions in 1867 and 1868 republicans?

Below are the choices the answer is D. 

A. The Republican Party was well liked and accepted among Southerners. B. White Democrats boycotted the elections to protest Congresss Reconstruction plan and the effort to give blacks the vote. C. The Republicans bribed blacks to support them, thereby gaining the upper hand at the conventions. D. President Johnson issued an executive order stating that Republicans had to be a certain percentage of the delegates.