Lincoln's main objective in the Civil War was to have a superior military to the Confederacy in order to preserve the Union. While he wanted a superior military, he did not want to lose the border states by announcing that he was against slavery, as this would hamper his goal towards building a military.
The answer is C. he was pro-slavery, as he eventually released the Emancipation Proclamation in 1863 that freed all slaves in the South.