The incorrectly spelt word in the given sentence is "scatered." The correct spelling is "scattered."

Here is the corrected sentence:

Reef-building corals are scattered throughout the tropical and subtropical western Atlantic and Indo-Pacific oceans, generally within 30°N and 30°S latitudes.

Explanation for the options:

• Subtropical: This is the correct spelling, referring to a region or climate zone that is intermediate between tropical and temperate.
• Reef: This is the correct spelling, referring to a ridge of rocks, sand, or coral that lies at or near the surface of the water.
• Latitudes: This is the correct spelling, referring to the angular distance of a place north or south of the Earth's equator.
• Scatered: This is the incorrectly spelt word. The correct spelling is "scattered," which means to be dispersed or spread out over an area.