Answer:
President Lincoln suggests emigration (Option C) as an alternative to slavery in his letter to McClernand. Emigration refers to the voluntary relocation of freed African Americans to suitable locations outside of the United States, with the goal of addressing racial tensions and conflicts arising from slavery. While sharecropping (Option A), apprenticeship (Option B), and education (Option D) were also relevant considerations, emigration was the specific alternative proposed by President Lincoln in his letter.