The correct answer to the question is option 3, the Tasman Sea. The Tasman Sea is the body of water that separates Australia from New Zealand.

Option 1, Cook Straits, is incorrect. Cook Strait is a narrow stretch of water that separates the North Island and the South Island of New Zealand, not Australia and New Zealand.

Option 2, Magellan, is also incorrect. Magellan refers to Ferdinand Magellan, a Portuguese explorer who is known for leading the first expedition to circumnavigate the world but does not relate to the water separating Australia and New Zealand.

Option 4, Great Barrier Reef, is also incorrect. The Great Barrier Reef is a coral reef system located off the coast of Australia and does not serve as a water boundary between Australia and New Zealand.

In conclusion, the correct answer is option 3, the Tasman Sea, as it is the water body that separates Australia and New Zealand.