Final answer:
New Zealand is the nation that is separated by the Cook Strait, which divides its North and South Islands.
Step-by-step explanation:
The nation that is separated by the Cook Strait, dividing its two main islands, is a) New Zealand.
The Cook Strait is a significant body of water that lies between the North and South Islands of New Zealand, making them the second- and third-largest islands in the Pacific Island region.
The two main islands of New Zealand comprise an area comparable to the US state of Colorado and are characterized by their distinct physical geographies, with the North Island featuring rugged coastlines and the South Island having a somewhat more regular coastline with deep fjords in the southern eastern parts.