Fluency is best defined as:

Fluency is best defined as the effortless flow of speech. This definition encompasses the smoothness and coherence in speech production that allows for clear communication. Fluency is not solely about the speed at which a person speaks; rather, it involves the ease and rhythm of speech and the natural integration of linguistic elements such as grammar and syntax.

In this context, the effortless aspect highlights that fluent speakers can articulate their thoughts without long pauses or interruptions, facilitating a more engaging and effective exchange of ideas. This ability contributes significantly to the listener's experience, as it ensures that the content is conveyed smoothly and understandably.

The other options do not accurately capture the essence of fluency. While speaking quickly without pauses might seem related to fluency, it does not account for the quality and coherence that fluent speech demands. Similarly, the use of complex vocabulary pertains more to language proficiency rather than to fluency itself. Lastly, maintaining eye contact is an important aspect of communication but does not directly relate to how fluent a person's speech is. Therefore, defining fluency as the effortless flow of speech accurately reflects the key characteristics of fluent communication.