What issue can overloaded transformer banks cause for customers?

Overloaded transformer banks can lead to voltage fluctuations, which can significantly affect the reliability of the power supply to customers. When a transformer bank is overloaded, it may not be able to supply a consistent voltage level to the connected loads. This instability manifests as voltage fluctuations, which can cause various problems for customers, such as flickering lights, inconsistent performance of electrical equipment, and potential damage to appliances.

In contrast, the other options do not accurately reflect the consequences of an overloaded transformer bank. Increased efficiency and improved power quality are desirable outcomes that typically result from well-maintained and appropriately loaded transformers, not from overload situations. Similarly, reduced costs do not apply here, as managing the effects of voltage fluctuations and possible equipment damage can lead to increased expenses for both consumers and utility providers. Thus, voltage fluctuations are the primary concern when it comes to overloaded transformer banks, making this the correct answer.