The sentence is comparing two methods for that the "seam allowance" can be "overcast" (no idea what that means
)
1. it can be done "as the seam is sewn" (a prepositional phrase)
or
2. it can be done "a separate process requiring deeper seem allowances" (a noun phrase)
Right now, those two methods are not parallel: "as the seam...instead of a separate process..." We need an answer choice that will create parallel form, but note that we can't affect the first portion ("as...") since it is before the underlined comparison term. Thus, we need an answer that will give us a preposition for the second phrase.
B works: "the seam allowance is overcast
as the seam...rather than
in a separate process"
C does not give us the "free" preposition we need. "In contrast with" is the comparison term itself.
D gives us a preposition as well, but saying "the allowance is overcast...to a separate process" is nonsense.
E is my favorite for sounding awesome, but it's wrong. The GMAT doesn't have a sense of humor
