What natural cue is thought to help female sea turtles return to their natal region to nest?

Answer: The local pattern of Earth's magnetic field

A trail of nutrients leaking from their old eggshells — Old eggshells do not maintain a nutrient trail across the ocean for returning adults. Embryos receive their food from yolk, and the proposed long-distance cue is the magnetic field, not nutrients from the beach.

Sand that stays at the same temperature as when they hatched — Beach sand does not stay at one temperature through the years a turtle spends at sea. Temperature affects development and nesting conditions, but it is not a permanent geographic address. Magnetic patterns provide a different kind of location cue.

The local pattern of Earth's magnetic field ✓ — Evidence supports the idea that turtles recognize geographic patterns in Earth's magnetic field. Remembering a natal magnetic signature could help them return to the right coastal region. Other local cues may help with the final approach, so this is not a claim of a perfect magnetic address for one exact nest.