Final answer:
A Radical Republican would likely criticize Lincoln for leniency in his inaugural address, as they desired harsher measures for the South and were advocates for African American rights.
Step-by-step explanation:
The question asks how a Radical Republican would respond to Abraham Lincoln's inaugural address in 1861. Given the historical context provided, Radical Republicans were critical of Lincoln's more moderate and conciliatory approach, particularly regarding the terms of reintegration of the South post-Civil War and the treatment of formerly enslaved people. Consequently, the most likely reaction from a Radical Republican to Lincoln's inaugural address would be to criticize Lincoln for leniency (Option C), as they favored stringent measures for the South and were staunch advocates for the rights of African Americans.