In the past several years, astronomers have detected more than 80 massive planets, most of them as large or larger than Jupiter, which circle other stars.

(A) most of them as large or larger than Jupiter, which circle
(B) most of them as large or larger than Jupiter and circling
(C) most of them at least as large as Jupiter, circling
(D) mostly at least as large as Jupiter, which circle
(E) mostly as large or larger than Jupiter, circling

This is not a large underline question. A fast and effecient way to solve questions that are small underlines is to prephrase and if that does not work, find grammatical errors.

Since I could not prephrase an answer in less than 10 seconds after reading the sentence. I started looking at the underlined part for grammatical errors, I know that as large should have as following it. Also the which should refer to the planets and not Jupiter (since which always refers to the noun just before it, in a few of the choices it is modifying Jupiter)

Looking at the choices and eliminating using which
A and D out.

As large as occurs in C only, also B can be eliminated because of the awkward wording and circling.
Thus as seen above C and E are good choices due to circling used rightly in them and only C has as large as which is needed by the sentence to be grammatically correct.

Thus pick C

The idea in SC should be to look at patterns and quickly eliminate what we know for sure is wrong. If one error is found in a sentence and you know for sure it is wrong, eliminate as many choices as you can using it, then find another error and eliminate the remaining contenders. Think about ideal ways a sentence can be written but do not spend an exorbitant amount of time doing that.