Final answer:
President Lincoln refused to accept the secession of the Confederate States and considered it unconstitutional. His focus evolved from solely preserving the Union to ending slavery in the rebellious states.
Step-by-step explanation:
The statement that best describes President Abraham Lincoln's reaction to the formation of the Confederate States of America is that he refused to accept the secession of states from the Union. In his inaugural address, Lincoln clearly stated that he believed the Union could not be dissolved by individual state actions, and he viewed secession as unconstitutional. Although Lincoln initially moved slowly on the issue of abolition, focusing on the Union's cohesion, he evolved toward a more definitive anti-slavery position as the Civil War progressed. By issuing the Emancipation Proclamation and pursuing policies for the reintegration of Southern states that accounted for the transition from a slave labor system to a free labor one, Lincoln signaled his commitment to ending slavery.