Maggots can be used to help predict PMI for a body discovered hidden among the tall weeds of field
twenty years after death
buried in a deep wet grave
on an ice flow off the Alaskan coast

The answer is A, hidden among the tall weeds of field.

Explanation:

Maggots can no longer grow on a body that died 20 years ago. It would have dried out, making it a bad habitat.

It can't be buried in a deep, wet grave, because it would be hard for flies or maggots to get that deep into the soil, so they're going to look for easier options.

If a body was found on an ice flow off the Alaskan coast, it is far too cold for flies to lay eggs on the body.

This leaves "hidden among the tall weeds of a field," which doesn't say that the body has dried out, it's probably not too cold, and it's easy accessible to the flies that need to lay eggs.

1. a tropical dry forest carries out more rapid nutrient cycling