Final answer:
The most likely group to agree with German's Rubicon reference would be A. John Calhoun and other politicians from the South.
Step-by-step explanation:
The student is asking which group of people would be most likely to agree with German's Rubicon reference. The Rubicon reference refers to the point of no return or a decisive step that cannot be reversed. Based on the provided information, the most likely group to agree with the Rubicon reference would be John Calhoun and other politicians from the South.
Thus, In the context of the information given, John Calhoun and other politicians from the South were opposed to typically compromise and saw northern actions at that time as more aggressive, which aligns with the idea of a decisive step.