(A) describes a part-to-whole flaw, which isn’t an assumption made by the author.

In this context, this would look like assuming that, because Classical Roman architecture is beautiful, rounded arches and symmetry must be beautiful. This flaw doesn’t happen in this argument.

The argument does not assume that drama is the most salient feature of postmodern architecture. The argument just says that postmodern architecture is dramatic. There is no comparison between “dramatic” and other features of postmodern architecture.

This is the flaw. The argument doesn’t acknowledge that it is possible to combine elements of classical Roman and postmodern architecture, and for the resulting style to lack beauty and drama.

This information is not relevant to the argument. We don’t need to know how the drama of postmodern architecture contributes to its beauty.

This is beyond the scope of the argument. The argument just discusses two architectural styles (Classical Roman and Postmodern) and what would happen if elements of these styles were combined. Other architectural styles are irrelevant to the argument.