This question will be very easy if you properly find the conclusion of the argument i.e. the second boldface. With that conclusion in mind, you can knock off 3 options.

Let's knock off 3 options,
A. the second is part of that explanation - It isn't an explanation/premise. The second boldface is a conclusion.
C. the second provides evidence in support of the conclusion - Again it is not the evidence. It is a conclusion.
D. the second acknowledges a consideration that weighs against that conclusion - Truly absurd!

We are left with B and E. Now look at the first boldface.

B. The first boldface is in support of the conclusion. By evaluating that fragments almost certainly contained no sulfur, we concluded that it is likely for a fragment to be large enough to avoid being burned up. B says that the first boldface is against the conclusion. Never!

So, E wins finally.

If we read the argument, the author is trying to state that sulphur contents are traced after the comet's entry. However, he is trying to make a point that comet itself does not have any sulphur, but origin of sulphur must be from other source.

Hence, in the last line, the author starts with "Since... " giving the reason and concludes with the statement " it is likely that ... ". We know that the last line is the conclusion.

As papgust mentioned, we can knock off A,C and D. In B, the first sentence is not weighing against the conclusion.

Regards,
Viju

E for me

last is the conclusion ... and first supports the conclusion by eliminating other explanation for sulphur content

I believe the correct answer should be E