Which of the following is a common defect in green coffee beans?

Mold is indeed a common defect in green coffee beans. It can develop during the harvesting, drying, and storage processes, particularly if the beans are exposed to moisture and poor ventilation. Mold can negatively impact the flavor and aroma of the coffee, leading to off-flavors that are undesirable in the final brewed product. This defect can be identified through visual inspection or by using sensory evaluation methods, as moldy beans often exhibit a distinct appearance or unpleasant smell.

In contrast, high acidity is a desirable trait in coffee, as it contributes to the complexity and brightness of the flavor profile. Excessive roasting can lead to defects, but this occurs during the roasting process rather than being a characteristic of green beans. High caffeine levels are not considered a defect; rather, they vary by coffee variety and are not indicative of quality in the context of defects. Therefore, mold is the most fitting answer regarding defects found in green coffee beans.