What is "vacillation" best described as?

The term "vacillation" refers to the act of repeatedly changing one's opinion or decision, indicating indecision or uncertainty. It embodies the notion of wavering, as someone who is vacillating cannot settle on a particular point of view or course of action and instead finds themselves fluctuating between different possibilities.

In this context, the other choices do not align with the meaning of "vacillation." Firmness in decision-making suggests a strong, unwavering stance, which is the opposite of vacillation. A sudden change in plans implies a decisive action rather than the indecisive nature of vacillation. An unwavering stance again contradicts the notion of going back and forth, as it denotes stability and certainty. Thus, "C. Wavering; going back and forth" accurately captures the essence of vacillation.