The best inference regarding Lincoln's use of the word "great" in this context is:
D. Lincoln thinks the civil war will be of outstanding significance or importance.
In the Gettysburg Address, Lincoln emphasizes the profound implications of the civil war for the nation and for the principles of liberty and equality. By referring to it as a "great civil war," he underscores not only the scale and intensity of the conflict but, more importantly, its critical importance in determining the future of the nation and its foundational values.