The prairie vole, a small North American grassland rodent, breeds year

We have to provide support to the claim:
The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.
We have to connect seasonal variation==> seasonal variation in mortality somehow (also note that the conclusion talks about "among young voles").
Winter = high
Spring through early autumn = low

A. It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.
This goes against the passage, who says that in spring the young voles should diminish.
B. Prairie vole populations vary dramatically in size from year to year.
By how much the size varies does not helps us in establishing our point.
C. The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.
Keep an eye on the conclusion we are trying to prove: our focus must be on the young voles.
We can infer something like "so the population is likely to increase in spring" reading C, which anyway is against the passage.
D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing.
Keep in mind that during winter the number are higher and during spring through early autumn the numbers are lower. If we know that despite harsh condition the population grows, we still do not have anything to connect YOUNG VOLES to the seasonal variation in mortality.
E. Snakes, a major predator of young prairie voles, are active only from spring through early autumn.
Look at E: it has all the key words (young prairie voles, spring through early autumn) and supports our conclusion stated above.

Snakes, a major predator of young prairie voles, are active only from spring through early autumn. => so during this period it feeds on young voles mainly, so we have connected
seasonal variation in group size ==> seasonal variation in mortality among young voles.