Which of the following statements regarding OSPF stub area is true?

A stub area in OSPF (Open Shortest Path First) is a specific type of area that is designed to minimize the amount of routing information exchanged. One of the key characteristics of a stub area is that it does not allow the advertisement of external routes that come from outside the OSPF Autonomous System. Instead, it can only receive a default route (0.0.0.0) from its neighboring areas.

Given this context, the statement that a virtual link cannot pass through a stub area is true. Stub areas are meant to limit the routing information and prevent the propagation of external routing information, which includes not allowing any OSPF virtual links to traverse them. This ensures that the stub area maintains its simplified design and operational efficiency.

In contrast, the other statements about AS-external-LSAs being advertised within a stub area, any area being a stub area, and external routes being purely based on a manually configured default route on an ABR, do not align with the fundamental design principles and restrictions of stub areas within OSPF. The purpose of restricting those types of advertisements is to enhance network stability and reduce routing overhead in those zones.