Final answer:
Alexander H. Stephens was the vice president of the Confederate States of America and took office on February 11, 1861, as a moderate figure alongside President Jefferson Davis. The option (D) is correct.
Step-by-step explanation:
The individual being described and who became the vice president of the Confederate States of America is Alexander H. Stephens. He was unanimously voted into office alongside President Jefferson Davis.
He took the oath of office on February 11, 1861. Stephens was not initially a pro-secessionist figure but was seen as someone who provided a sense of moderation to the Confederate government. His selection as vice president was strategic, aiming to project a more moderate image for the newly formed Confederacy. Therefore, option (D) is correct.