Final answer:
The Radical Republicans opposed President Lincoln's Reconstruction plan because it was too lenient, offering amnesty to many Confederates. They wanted to enforce stricter conditions and protect the rights of former slaves.
Step-by-step explanation:
The major reason the Radical Republicans opposed President Abraham Lincoln's Reconstruction plan was D. the plan offered amnesty to nearly all Confederates who would swear allegiance to the United States.
The Radical Republicans believed that Lincoln's Ten Percent Plan was too lenient as it only required 10% of the 1860 voters in Confederate states to take an oath of allegiance to the Union. They sought more stringent measures, wanting to remake the South and ensure protection for formerly enslaved people. Their plan included harsher punishments for Confederate states and pushed for greater reforms that went beyond what President Lincoln had proposed.