First, let us refresh some basics, lest we go astray later. A pronoun, whether possessive or reflexive or a simple standard one, should refer to only one entity in any given sentence.
A pronoun's first priority is to refer to the subject of the clause, and if there is no appropriate subject for referring in the main clause, then can refer to the proximate noun, be it an object or object of the preposition and so on.
Here, the floor of the church is the subject and the pronoun it or its or itself should refer to the floor of the church first. But it turns out to be incongruous since the floor itself cannot be the floor's own attraction. Hence, we have to seek the church, the nearest noun, which also happens to be the object of the preposition 'of' as the legal referent for it/its/itself.
Constructed with the finest Italian marble, the floor of the church is its greatest attraction and is more attractive to tourists than are its ornate stained-glass windows.
(A) attraction and is more attractive to tourists than are its ornate stained-glass windows -- This passes the test, if we take the church to be the most befitting antecedent for the pronoun 'its' in both the places.
(B) attraction, itself more attractive to tourists than are its ornate stained-glass windows -- neither the floor nor the church can fit in since 'itself' refers to the floor and 'its' refers to the church. itself more attratvie than are is not parallel. we must cut off the 'are'
(C) attraction, itself more attractive to tourists as are its ornate stained-glass windows -- same problem as in B in addition to the ungrammatical idiom 'more… as'
(D) attraction, being more attractive to tourists than its ornate stained-glass windows are--- first problem is the use of being which is denounced as superfluous in GMAT.
(E) attraction as it more is attractive to tourists than are the church's stained- glass windows --- I don't see a valid sentence here. what does the phrase "as it more is" mean? Secondly, the first 'its' refers to the church's, but the second 'it' refers to the floor.
A perhaps is the best fit.