There is a difference between beauty and truth. After all, i

- the most realistic pieces are the most truthful.
- if there were no difference between beauty and truth, then the most realistic pieces of art would be the best as well.
- But many of the most realistic artworks are not among the best.

Conclusion: There is a difference between beauty and truth.

Note that the premises talk about "realistic pieces being truthful but not among the best". And then conclusion says that there is a difference between beauty and truth.
The argument does not link "best" with "beautiful". These are two different concepts. To conclude about "beautiful", we need to talk about how "best" and "beautiful" are the same.

So the assumption will link best with beautiful.

(A) The most beautiful artworks are the best artworks.

This is it. It says that beautiful and best are the same.

(B) If an artwork contains nonrealistic elements, then it is not at all truthful.

Not true. We don't need to assume that if there are any nonrealistic elements, then it is not at all truthful.

(C) None of the best artworks are realistic.

All that the argument says is that many of the realistic ones are not among the best.

(D) Only the best artworks are beautiful.

Not necessary. Best needs to be beautiful but ONLY best should be beautiful is not necessary.

(E) An artwork’s beauty is inherently subjective and depends on who is viewing it.

Irrelevant.

Answer (A)