Option 1, Chain viper, is not the correct answer. Chain vipers, like most snakes, do not build nests. They lay their eggs in a hidden or protected location.

Option 2, King Cobra, is the correct answer. King Cobras are unique among snakes in that the female builds a nest for her eggs, providing protection until the young hatch.

Option 3, Krait, is not the correct answer. Kraits are known for their potent venom, not their nesting behavior. They typically hide their eggs in cracks, crevices, or burrows, but do not construct a nest.

Option 4, Saw-scaled viper, is also not the correct answer. While they are known for their distinctive scales and aggressive behavior, saw-scaled vipers do not build nests. Like most snakes, they lay their eggs in a hidden, secure location.

So, the unique behavior of nest building is only found in King Cobras among these given options.