Some of our federal legislators are opposed to government

Ok then. Let's break it down.
Premises:
- Some of our federal legislators believe that much of modern photography portrays nudity and is thus obscene.
- Even they would agree that Michelangelo's works of art, most of which depict nudity, are not obscene.
Conclusion:
These federal legislators should not consider modern photography obscene. (He says they are mistaken so basically he is saying that they should not consider it obscene.)

Now think of what logic is being used in the argument above. It simply says:
A has B and is thus considered C.
D also has B but is still not considered C.
So we should not consider A C.

A - Modern photography
B - Nudity
C - Obscene
D - Mich. art

What is the flaw here?
According to the argument, if there is nudity in art it is considered obscene. The reason why Mich's work is not considered obscene in spite of having nudity could be that his work has something else too.

Let me explain this with another example:
My argument:
You have a pen and thus can write.
I have a pen but I cannot write.
So you cannot write either.
What can weaken my argument? How about: I have a broken arm because of which I cannot write in spite of having a pen.

Option (E) tells you that Michelangelo's work has high artistic value. That is why it is not considered obscene in spite of having nudity. This weakens the argument since Michelangelo's work has something extra and the same may not be applicable to modern photography.

Option (D) needn't be considered because general citizenry is not a part of the argument at all. The author is trying to convince the legislators that modern photography should not be considered obscene.

Option (A) has a disconnect with the argument. It says that 'The artistic level of the works of the vast majority of modern photographers does not approach that of Michelangelo's works.' but it doesn't say that Michelangelo's work is not considered obscene because of its high artistic value. It is as if you presented a difference between the two - modern photography and Mich's work but did not say how this difference impacts the argument. e.g. in the example above, if I say, "Your arm is absolutely fine unlike mine which is broken." it is not enough because I haven't said that because of my broken arm, I cannot write. (Remember, I have to mention this to present the relationship. What if my non-writing arm is broken.)

Now, I hope it makes more sense why the answer is (E).