In which scenario would a totally stubby area be used in OSPF?

A totally stubby area in OSPF is specifically designed to limit the types of routes that are propagated to and from the area. By definition, a totally stubby area does not allow for the propagation of external routes (routes from outside the OSPF domain) and does not permit summary routes from other areas. This characteristic is essential for reducing the size of the OSPF routing table and minimizing routing overhead.

In a totally stubby area, the only route that is advertised is a default route, which facilitates simpler routing within the area while keeping the routing information minimal. This is particularly useful in scenarios where you want to maintain streamlined and efficient routing within an area that does not require knowledge of external networks.

While considering the other scenarios, it becomes clear that they do not align with the definition and purpose of a totally stubby area. The option that mentions allowing external routes or summary routes contradicts the fundamental characteristics of such areas. Thus, the correct choice accurately reflects the primary function of a totally stubby area in preventing the advertisement of external routes.