South Korea has witnessed the world's most dramatic growth of Christian congregations; church membership is expanding by 6.6 percent a year, fully two-thirds of the growth coming from conversions rather than the population increasing.
(A) coming from conversions rather than the population increasing
(B) coming from conversions rather than increases in the population
(C) coming from conversions instead of the population's increasing
(D) is from conversions instead of population increases
(E) is from conversions rather than increasing the population.

Hey folks,

When we have a comparison (here, signaled by "rather than"), we need to make sure we are comparing two parallel things: a noun to a noun, a verb to a verb, etc.

(A) coming from conversions rather than the population increasing - noun to a verb, eliminate
(B) coming from conversions rather than increases in the population - noun to a noun, keep
(C) coming from conversions instead of the population's increasing - noun to a verb, eliminate
(D) is from conversions instead of population increases - noun to a noun, keep
(E) is from conversions rather than increasing the population - noun to a verb, eliminate

Then to decide between B and D, we should notice that D incorporates a verb ("is") that isn't present in B. This gives us both a subject and a verb after the comma between "year" and "fully". Having both a subject and a verb often indicates an independent clause - something that can stand alone as a sentence. Two independent clauses can be separated by a period, a semi-colon, a colon, a dash, and a comma + conjunction, but NEVER by only a comma.

So here we have
"South Korea has witnessed the world's most dramatic growth of Christian congregations."
semi-colon
"Church membership is expanding by 6.6 percent a year."
comma
"Fully two-thirds of the growth is from conversions instead of population increases."

So we have three independent clauses here, since all of these can stand on their own as sentences. However, the last two are separated with only a comma, giving a comma splice, which is a massive sentence structure error.

So we can eliminate D, leaving us with B.