Which word best completes the phrase about understanding the speaker?

The word "enough" in the context of understanding the speaker conveys the idea of sufficiency; it indicates that the level or quality of understanding is adequate. In conversations or speeches, saying one understands "enough" implies that the listener has grasped the essential points or meanings conveyed by the speaker. This usage is particularly relevant when discussing comprehension, as it highlights that the listener may not need complete or full understanding of every detail, but rather a satisfactory grasp of the main ideas.

The other options, while related to understanding, do not fit as seamlessly in this context. "Clearly" suggests the manner in which something is understood, which is more about the quality of clarity, rather than the degree of understanding. "Quickly" relates to the speed of understanding, which does not inherently reflect the quality of comprehension. "Easily" pertains to the simplicity of understanding, indicating that something is straightforward, but it doesn’t address whether the understanding is sufficient or complete. Thus, "enough" stands out as the most appropriate choice in this phrase.