This question asks us to find the answer that does NOT help to explain why scientists found older fossils of warm-weather beetles than of warm-weather pollen. This means we can eliminate any answers that help to explain why the warm-weather beetle fossils would be older.

(A) Cold-weather beetle fossils can be mistaken for those of beetles that live in warm climates.
If the older beetle fossils were actually cold-weather beetles, this would indicate that the warm climate may not have developed as early as the beetle record indicates. So the information from the beetle record may be incorrect, which would explain the difference. Eliminate.

(B) Warm-weather plants cannot establish themselves as quickly as can beetles in a new environment.
This indicates that while warm-weather beetles could have shown up as soon as the weather warmed up, plants might have taken a while. This would mean that pollen would not have shown up in the fossil record until later than beetles would have, which would explain the difference. Eliminate.

(C) Beetles can survive in a relatively barren postglacial area by scavenging.
This would indicate that beetles can survive pretty much right after the climate warmed up. Plants, on the other hand, might need a less barren landscape, which means they (and their pollen) wouldn't have shown up until later, which would explain the difference. Eliminate.

(D) Since planes spread unevenly in a new climate, researchers can mistake gaps in the pollen record as evidence of no new overall growth.
This indicates that plants could have been growing without leaving any evidence in the pollen record. This would mean that the information from the pollen record may be incorrectly interpreted, which would explain the difference. Eliminate.

(E) Beetles are among the oldest insect species and are much older than many warm-weather plants.
We know that beetles and warm-weather plants both "returned" after the weather warmed up - in other words, they both existed before the weather warmed up. So it doesn't really matter how old either of them were. What matters is when they "returned" to the area - either the older beetles or the younger plants could have shown up first! E doesn't explain anything, so we can eliminate.