Understanding the argument -
Walter: A copy of an artwork should be worth exactly what the original is worth if the two works are visually indistinguishable. - Opinion. It's based on the assumption that if they are "visually indistinguishable," then they possess all the qualities.
After all, if the two works are visually indistinguishable, they have all the same qualities, and if they have all the same qualities, their prices should be equal. - This is a conditional. Walter asserts that "being visually indistinguishable" is a sufficient condition or enough for the two works to have all the same qualities. Meaning "being visually indistinguishable" 100% guarantees "all the same qualities." See the usage of Zero conditional to express the universal truths or facts.
Marissa: How little you understand art! Even if someone could make a perfect copy that is visually indistinguishable from the original, the copy would have a different history and hence not have all the same qualities as the original. - Marissa challenges the "sufficiency" condition. She says even if "they are visually indistinguishable," it doesn't guarantee 100% that "they possess all qualities." She substantiates this by saying that "they could have a different history." Maybe the original one is 500 years old and has gone through the test of time, while the copy is just one year old and has been kept in all the controlled environments.
Option Elimination - Flaw
(A) Whether a copy of an artwork could ever be visually indistinguishable from the original - It can be, and that is not an issue between Marissa and Walter. Distortion.
(B) Whether the reproduction of a work of art is ever worth more than the original is worth - "worth more than the original" is out of scope.
(C) Whether a copy of a work of art is ever mistaken for the original - It can be, and that is not an issue between Marissa and Walter. Distortion.
(D) Whether a copy of a work of art could have all the same qualities as the original - yes. While Walter believes that is the case, Marissa challenges that aspect.
(E) Whether originality is the only valuable attribute that a work of art can possess - "originality" as a minimum condition for it to be valuable is not disputed. This is out of scope.