Question

First of all, we are not talking of Lincoln and Douglass as individuals here but together. In other words, the clause in the middle (separated by commas) is including both men in black history.

Hence, the correct word here won't be each but both.

On the basis of this, A and D are eliminated.

Now, we need a conjunction after the clause as well as a connection as to why black history is connected to the two people's birth month.

Because is the conjunction that best fits that Rolland clearly connects the proceeding clause with "both men were...... Of slavery".

So, C is the correct option.

IMO C!

Nice explanation,
How I solved this question is as follows:

A they - ambiguous
B who – refers to birth
C doesn’t have any pronouns for ambiguity.
D “each of whom” awkward
E doesn’t complete the sentence logically.