I think there is a problem with "as". It is redundant.
And also it is concise to use its rather than "church's"

hope this helps!

E seems to be the correct answer choice.

A is grammatically correct, but the use of 'it' is ambiguous. We can't be sure what 'it' refers to: the church? the floor?
B is awkward and ambiguous - the use of 'itself' is not clear - what does it modify?
C uses the comparative 'as' incorrectly
D repeats the ambiguity of 'its'
E - corrects the ambiguity be replacing 'its' with 'the church's'and clarifying the reference. E also improves the logic of the sentence, by using the causal phrase "as it is more".

Explanation by book:

64. [spoiler](B)[/spoiler]
The final segment of the sentence, is more attractive to tourists than... should be a subordinate phrase that modifies the floor. The original sentence is not overtly wrong, but since the final segment provides more information about the attractiveness of the floor, it should technically modify the clause the floor of the church is its greatest attraction instead of existing as a separate, redundant phrase. The original and suggests that the popularity of the floor and its appeal for tourists are separate ideas. Since they're related, the structure of the sentence should tie them together more clearly. Choices (Ð’), (C), and (D) all subordinate the final phrase. Eliminate (C) because the proper idiom is more...than not more...as. Eliminate (D) because being is fully unnecessary. (B) it is.