Which of the following is NOT considered a type of non-regulatory airspace?

Class D airspace is indeed a type of regulatory airspace, primarily established around airports that have a control tower. It is subject to specific rules and management by air traffic control, with defined boundaries and operational requirements that must be adhered to by pilots. This distinguishes it from non-regulatory airspace, which includes areas that do not have such stringent requirements.

In contrast, alert areas, military operations areas, and controlled firing areas are considered types of non-regulatory airspace. Alert areas are locations where there may be a high volume of pilot training or unusual aerial activity, but they are not controlled by air traffic services. Military operations areas are designated for military training activities and, while they are marked to inform civilian pilots, they do not impose regulatory constraints like Class D airspace. Controlled firing areas are designated for activities that could be hazardous to non-participating aircraft, but they do not require regulations enforced by air traffic control since flight activity can be suspended during active operations.

Therefore, the identification of Class D airspace as regulatory precisely underscores why it is the correct answer in distinguishing regulatory from non-regulatory types of airspace.