"The purpose of correctly setting pen weights and colors is..."
I just don't like this one; none of the answers is objectively the best, and they could all be defended.
"Look good on the screen" is unclear. Is this an artistic evaluation? Pencolors in AC are good when they clarify the model. But that's my opinion. "Improve the graphic appearance" is best, but it makes it sound subjective again, and it raises the question of whether we're talking about AC or PM. "Viewing the plan easier on the eye". Where, on the paper? That's certainly part of it. Or in AC? Meaning the colors shouldn't clash? "Better simulate hand-drawings". If you are of the "Mr. Hand" school (I am not), this would be a consideration, though I would argue pens weights alone don't do it. This answer is correct if we interpret "simulate hand-drawings" to mean "maintain traditional drafting standards".
Pens in PM are for putting good drafting on the paper. Pen settings in AC are for "previewing" the drafting, as well as intelligently presenting the project to the user. Reasonable people may disagree, so multiple choice isn't the best format such a question.