One half of the Greenvale Bank's tellers work full time,

OA is (C)
Explain please?

(A) One half of the Greenvale Bank's tellers work full time, compared with Bosc Bank's two thirds and Community Bank, Farlo Bank, and Jantex Bank's three quarters.
Comparing GB's tellers with BB's two thirds and etc.

(B) One half of Greenvale Bank's tellers work full time, in Bosc Bank this number is two thirds, and in Community Bank, Farlo Bank, and Jantex Bank the number is three quarters.
The reference of "this number" is unclear.

(C) In Greenvale bank, one half of the tellers work full time, compared with two thirds in Bosc Bank and three quarters in Community Bank, Farlo Bank, and Jantex Bank.
Fits parallelism

(D) The fraction of tellers in Greenvale Bank who work full time is one half, compared with Bosc Bank's two thirds, Community Bank's three quarters, Farlo Bank's three quarters, and Jantex Bank's three quarters.
It would be more parallel if it was "two thirds in Bosc Bank", instead of "BB's two thirds".

(E) The fraction of Greenvale Bank's tellers working full time is one half, that of Bosc Bank is two thirds, and that of Community Bank, Farlo Bank, and Jantex Bank is three quarters.
It would be more parallel if it was "that of BB's tellers ...". Can't compare a fraction of GB's tellers with a fraction of a bank.

(C) Agree with HongHu..

Only (C) maintains parallelism.. (in XY bank)

We can also refute (D) and (E) for wordiness.

but is "compared with" the correct idiom? I always thought it was "compared to"

Both are idomatic usages, I believe. Compared with is used for things that are similar; compared to is used for things that are not similar.

(C) In Greenvale bank, one half of the tellers work full time, compared with two thirds in Bosc Bank and three quarters in Community Bank, Farlo Bank, and Jantex Bank.

The best way to figure this one out is to look at what is being compared. Here, it is the number of tellers - which is pretty obvious. The next step is find the prepositional phrase applies to each figure, which in this case is "of the tellers." None of the other sentences are structured to allow for this.

just wanted to share the usage of compare with and compare to(as per Kaplan)

"On the GMAT, Compare with is the generally preferred form. Use compare to to point out an abstract or figurative likeness, and compare with to consider likenesses(is this usage correct????) and differences in general"