Final answer:
Customers complaining might display emotions such as anger, disgust, contempt, and sadness, each identifiable through various facial expressions and verbal cues.
Step-by-step explanation:
When customers are complaining, they may display a range of emotions. Four possible emotions that one might encounter include anger, which can be expressed through frowns or an elevated tone of voice; disgust, evident when the customer expresses extreme distaste or aversion to a situation; contempt, which may present as smirking or rolling of the eyes, signaling a feeling of disdain; and sadness, where the customer may look downcast or tearful, showing their disappointment.