Which of the following is a cause of external obsolescence?

External obsolescence refers to a loss in property value due to factors that are external to the property itself. This type of depreciation typically arises from changes or influences in the surrounding environment that negatively impact the property's desirability or usability.

When considering the correct answer, changes outside the property boundaries can include a variety of factors such as shifts in zoning regulations, increased crime rates in the area, the construction of undesirable facilities nearby, or economic downturns affecting the neighborhood. All of these can lead to a decrease in property values because they detrimentally affect the market's perception of that location.

On the other hand, the other options pertain to issues that are more closely related to the property's own condition or construction. Deferred maintenance and a malfunctioning HVAC system represent issues that occur within the property itself and are typically classified as physical obsolescence rather than external. Errors in construction materials can also be viewed as a form of physical inadequacy that affects the property's structural integrity and value but does not stem from external influences. Thus, these factors are not considered causes of external obsolescence.