b), so discard a token or play an action on the base card. It's in the rules, page 12:
If a Player enters into an Agreement with a Defensive Location
(Locations with a brown field where the blue Agreement field
normally is) he pays the cost of the Cooperation Action (by
discarding Cooperation Contact tokens with a total sum no less
than the Distance of the Location) as normal. Then the Defensive
Location Card is placed under another Location Card
already belonging to that Player’s State so that the brown field
From this moment on the Location with the Defensive Location
attached has its Defense Distance increased by the number
on the brown field of the Defensive Location. This change only
applies in case of Conquest, not in the case of starting Cooperation
by another Player.
Note: It is not possible to initiate Cooperation with Defensive Locations
belonging to other Players.
Card” is put under Base.