A slow decompression can occur due to which of the following?

A slow decompression occurs when there is a gradual decrease in the pressure differential between the inside of an aircraft and the external environment. A leak from a failing seal is a primary cause of this phenomenon, as it allows the pressurized cabin air to escape slowly over time, rather than all at once. This can lead to hypoxia and other physiological effects, as the body may not acclimatize properly to the changing pressure levels.

In contrast, rapid altitude gain typically results in a swift change in cabin pressure, leading to rapid decompression. Over-pressurization might cause structural damage, but does not lead to a situation of decompression in the traditional sense. An emergency descent is intended to quickly return to safe altitudes and would generally not lead to slow decompression, rather it may cause rapid decompression if there is a need to quickly drop altitude. The nature of a leak from a failing seal perfectly encapsulates the slow and progressive nature of this type of decompression, making it the correct answer.