Final answer:
Proficiency or fluent behavior cannot be assessed solely by percentage because it requires considering both speed and precision, which percentages alone do not capture.
Step-by-step explanation:
The student's question is about what cannot be assessed solely by percentage when looking at the development of proficient or fluent behavior because such an assessment requires reference to additional factors. The correct completion to the statement in the question is 'reference to speed and precision,' which corresponds to option c. This is because proficiency in a behavior or skill typically involves being able to perform that skill both quickly (speed) and correctly (precision), aspects that percentages alone may not fully capture.
In the context of development, percentages can provide valuable information regarding the proportional accuracy or frequency of a particular behavior, but it does not give us a complete picture of skill mastery. For instance, in measuring the development of an athlete's sprinting ability, we would consider not just the percentage of successful sprints but also how fast (speed) and how consistently accurate (precision) the athlete performs over time.