The OSPF database of a router shows LSA types 1, 2, 7, and 3 default router only. Which type of area is this router connected to?

A stub area allows Type 1, 2, and 3 LSAs but does not allow Type 7 or Type 5 LSAs into the area, instead typically injecting only a default route as a Type 3 LSA.

A totally stubby area allows only Type 1 and 2 LSAs and a default route (Type 3 LSA generated by the ABR), explicitly blocking all other Type 3, 4, and 5 LSAs, and does not allow Type 7.

An NSSA totally stubby area allows Type 1, 2, and 7 LSAs but, like a totally stubby area, it only allows a default route as a Type 3 LSA, blocking all other Type 3, 4, and 5 LSAs from entering the area. While it contains Type 7s, the '3 default router only' could be seen as aligning, but NSSA (D) is the broader category that encompasses this scenario if no other Type 3s are currently present.

A Not-So-Stubby Area (NSSA) allows the injection of external routes (from another routing protocol) into OSPF via a Type 7 LSA, which is then translated by the ABR into a Type 5 LSA for distribution to other areas. The presence of Type 1 (Router) and Type 2 (Network) LSAs is standard for any OSPF area, and a Type 3 default route can be propagated into an NSSA, consistent with the observed LSA types.