This has no clear impact on the argument. Other figures of similar ages doesn’t suggest that the same artist who painted the self-portrait would or would not have painted the battle scene.

So, most figures in the battle scene painting look like real people. But the artist who painted the self portrait was also a real person, so there’s nothing in (B) suggesting that artist wouldn’t have included a representation of himself in the battle scene.

This says that painters used human references, but that doesn’t mean the painted figures were full-on portraits of the models. (The models could have just been used to help painters draw body shapes or positions accurately.) Even if we assume that the painted figures were outright portraits of the models, for (C) to weaken the argument, we’d then have to assume that the painter in the self-portrait was a model for the battle scene. And we have no reason to believe the painter was also a model.

This provides a reason to think the painter would not have included himself in the battle scene painting: including himself would have violated etiquette. Yes, we need to assume that the painter followed etiquette for (D) to weaken the argument. But this is a less arbitrary assumption to rely on than what the other answer choices would require. By definition, etiquette is customary. It’s something that society pressures people to conform to. So while (D) doesn’t powerfully weaken the argument, it does weaken.

Painters can paint scenes of battles that occurred many years ago. We have no reason to think the year the historic battle occurred has to be the same year the painting of that battle was made.