The correct answer is option (C):
Star
The relationship in the first word pair, Archipelago : Island, is a part-to-whole relationship. An archipelago is a group of islands. We need to find a word in the options that has the same relationship to the second word in our new pair.
Looking at the options:
* Garden: A garden is a place where flowers, among other things, might be found. This doesn't represent a group of something, but a location.
* Flower: A flower is an individual object, and isn't a collection.
* Star: A constellation is a group of stars. This mirrors the archipelago-island relationship, being a group (constellation) of individual elements (stars).
* Valley: A valley is a geographical feature, not a collection.
* Mountain: A mountain is an individual object, and isn't a collection.
Therefore, the correct answer is Star because a constellation is a group of stars, just like an archipelago is a group of islands.