Is it D?

...., the known temperature tolerances of the existing beetle species were used to determine

I'll pick E.

I agree with E.

1) By examining fossilized beetles a research team has produced the most detailed description of temperatures in Britain over the past 22,000 years.
2) Fossils of species that still exist were selected and dated.
3) When individuals of several species found in the same place were found to date to the same period, the known temperature tolerances of the existing beetle species were used to determine the maximum summer temperature that could have existed at that place and period. <-- conclusion

We need an assumption to hold up the conclusion made by the reseach team.

The procedure of the researchers assumes which one of the following?

(A) Beetles can tolerate warm weather better than cold weather.
- Out. It does not fill in the large gap between matching the temperature tolerance of existing beetle species and those of the fossilised species.

(B) Fossils of different species found in the same place belonged to different periods.
- Again, it doesn't hold up the conclusion.

(C) The process of dating is more accurate for beetles than for other organisms.
- Out. Accuracy doesn't mean that what is true for earlier speicies must be true for latter species (temperature dependence)

(D) The highest actual summer temperature at a place and period equaled the average of the highest temperatures that could have been tolerated by each of the beetle species found there and dated to that period.
- Out.

(E) The temperature tolerances of the beetle species did not change significantly during the 22,000-year period.
- This should be the assumption. The conclusion made depends on the temperature tolerance of beetles today. If there was a large variation, then the data would not be accurate.

E it is.

E ...

E too