(A) We know info is extracted from current glaciers, we can't infer anything about "glacial periods".
(B) No, glacial evidence tell us the TYPE of climate, whereas carbon dating tell us WHEN that climate was prevalent.
(C) We don't know whether similarities exist, we only know the climates of the different periods can be compared.
(D) We don't know how "continuous" pollen is, we don't know how often we find it in the glaciers.

(E) By POE, this is the only choice left. We know we can determine climate by examining frozen prehistoric pollen from plants, so certain plants must be reliable indicators of what type of climate/weather existed in a given region.

Option A is too much of an inference, the passage does not discuss the possibility of several glacial periods. It only discusses that weather conditions might have been different.
Option C is too strong by saying that similarities do not exist at all. The passage only discusses how to compare weather conditions using pollen data.

Having eliminated two out of the three options given, I guess, I'll go with B. The additional premise that starts with "Furthermore" is a good indication that we are flowing in the proper direction.
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Lets solve this CR with the CR Bible strategies:

The structure of the argument is:
Premise + Conclusion + Additional Premise
(One of the more, by comparing) + (scientists can figure out) + furthermore

So, we can rearrnage the parts as:
Premise + Additional Premise + Conclusion

Premise: One of the more reliable methods of determining regional climatic conditions in prehistoric periods is to examine plant pollen trapped in glacial ice during ancient times.

Additional Premise: Furthermore, by submitting the prehistoric samples to radiocarbon dating techniques, we can also determine when certain climatic conditions were prevalent in that portion of the globe.

Conclusion:
Scientists can figure out approximately what the weather was like at the time of pollen deposition.

We can say an inference is like a Must be True statement based on the stimulus given. So, any answer choice rephrasing any of the above premise/additional premise/conclusion and avoiding new info will be the correct answer.

Now, lets check the given options:

(A) The earth has undergone several glacial periods. [No mention of Several glaciers. [New info based option. Incorrect]

(B) Radiocarbon dating can be corroborated by glacial evidence. [This is a Reverse case trap. Radiocarbon dating and glacial evidence are stated in the stimulus but in different context. Incorrect]

(C) Similarities between prehistoric and contemporary climates do not exist.
[This is Opposite case. Read the conclusion and additional premise. Incorrect]

(D) Pollen deposition is a fairly continuous process.
[No mention of such process. New info. Incorrect]]

(E) Certain flora are reliably associated with particular climatic conditions.
This is just the prephased version of the conclusion. Correct.

So, OA is E.
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+1 E

Choice E is correct because of this premise in the argument:

"By comparing such pollen samples with spores taken from modern vegetation, scientists can figure out approximately what the weather was like at the time of pollen deposition".
So, certain type of polen is related with some climatic conditions.
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