IMO E

attraction and is more attractive to tourists than are its ornate stained-glass windows

Its no clear antecedent.
Similarly D also uses Its so it is also out

C. attraction, itself more attractive to tourists as are its ornate stained-glass windows.

Changes the meaning

Itself unidiomatic

Similarly B is also gone


Left with E

grammatically Correct and proper pronoun reference


Hope this is clear

IMO A
Constructed with the finest Italian marble, the floor of the church is its greatest attraction

Parallelism, Logical predication and Idiom usage are tested
A. and is more attractive to tourists than are its ornate stained-glass windows --- the floor of church is X and is Y; the floor of church is more attractive to tourists than are its ornate stained glass.

B. attraction, itself more attractive to tourists than are its ornate stained-glass windows --- itself wrongly modifies attraction

C. attraction, itself more attractive to tourists as are its ornate stained-glass windows. --- itself wrongly modifies attraction, more X as Y is wrong idiom usage


D. attraction, being more attractive to tourists than its ornate stained-glass windows --- attraction is modified; X being more attractive than Y is wrong idiom

E. attraction as it more is attractive to tourists than are the church's stained glass windows --- as it more is attractive is awkard