With which type of IS-IS router can an IS-IS Level 1-only router form an adjacency?

A.
any IS-IS Level 1 router of any area

B.
any IS-IS Level 2 router of any area

C.
any properly configured IS-IS router

D.
any IS-IS Level 1 router of the same area

Explanation:
In IS-IS, a single AS can be divided into smaller groups referred to as areas. Routing between areas is organized hierarchically, allowing a domain to be divided administratively into smaller areas. IS-IS accomplishes this organization by configuring Level 1 and Level 2 ISs. Level 1 systems route within an area, and Level 2 ISs route between areas and toward other ASs. A Level 1 and Level 2 system routes within an area on one interface and between areas on another.

A Level 1 and Level 2 system sets the attached bit in the Level 1 PDUs that it generates into a Level 1 area to indicate that it is a Level 2-attached backbone router and that it can be used to reach prefixes outside the Level 1 area. Level 1 routers create a default route for interarea prefixes, which points to the closest (in terms of metrics) Level 1 and Level 2-attached router.

http://www.debugall.co.uk/2009/01/03/isis-level-1-level-2-areas-isis-redistribution-and-isis-default-route-origination/