Final answer:
The central idea in the excerpt from "White House Funeral Sermon for Abraham Lincoln" by Dr. Phineas D. Gurley is that the cause that Lincoln fought for will survive.
Step-by-step explanation:
The central idea in the excerpt from "White House Funeral Sermon for Abraham Lincoln" by Dr. Phineas D. Gurley is b. The cause that Lincoln fought for will survive.
In the sermon, Dr. Gurley discusses how Lincoln faithfully and patiently worked on a cause, and that cause will continue to exist even after his death. This central idea is supported by the following statement in the excerpt: "[Lincoln] had securely fastened the car of state to the track upon which he had set it, and the great and beneficent cause in which he labored, he knew, could not ultimately fail."