Not necessary, because (A) arguably weakens the argument if assumed. If Blue Irises is nonrepresentational, then it might be abstract art, in which case it might be part of our city’s art movement.

Not necessary, because we already know it doesn’t show a sharp contrast of light and shadow. Whether this is because it doesn’t depict shadows or for some other reason doesn’t affect the reasoning. What matters is whether Blue Irises is an abstract painting.

Necessary, because if it were not true — if Blue Irises IS an abstract painting — then it is among the paintings that could be part of our city’s art movement without having a sharp contrast of light and shadow.

Not necessary, because the argument concerns whether paintings are part of the city’s art movement — this doesn’t require any belief about all nonrepresentational paintings “ever produced in the city.” The concept of being produced in the city is different from being part of the city’s art movement. In any case, all that the author says about all nonrepresentational paintings is that they do not depict light or shadow. This doesn’t commit the author to thinking they have bold brushwork.

Not necessary, because the argument concerns what is required in order to be part of the city’s art movement. One of the potential requirements is having both bold brushwork and a sharp contrast of light and shadow. But this doesn’t commit the author to thinking that every painting with sharp contrast of light and shadow is part of the city’s art movement. We don’t know, for example, whether those paintings all have bold brushwork.