Final answer:
The answer to the question is A. Variability, which refers to the unpredictable nature of customer arrival and service times that can affect queue formation even when capacity exceeds demand.
Step-by-step explanation:
Capacity exceeding demand does not guarantee that a queue will form because there can be variability in how customers arrive and how long it takes to serve them. The correct answer is A. Variability. This concept is important in understanding the dynamics of service systems, where factors such as arrival rate and service rate play a crucial role. For example, a model might assume that customers arrive one at a time and at a consistent rate; however, in practice, arrivals can fluctuate, with groups of customers arriving at once or at different times during the day. Additionally, service times can also vary, which affects the overall flow and potential queuing of customers.