Marta does not cast doubt on anything James has said in his premises. In fact, she concedes that the technique James mentions is effective, and she doesn't say anything to suggest she doesn't believe catalytic converters work as James says they do. She just adds a piece of information to what James has said, which James hasn't considered: despite all the likely benefits of this technique, it hasn't yet been adopted in the manufacturing industry.

Neither James nor Marta talks about advocates of nuclear power. We can't assume James himself is necessarily such an advocate. He just mentions one potential benefit of a specific technique related to nuclear power — he doesn't advocate for nuclear power overall. Besides, Marta doesn't question James's credibility, or anyone else's. She just mentions a piece of information James hasn't considered.

This answer choice is worded confusingly, but it is saying that Marta accuses James of circular reasoning — this is what it means for an argument to assume the truth of its conclusion. But James's argument is not circular. His conclusion — nuclear power is contributing in at least one way to a cleaner environment — is based on premises that are separate from his conclusion, and don't require his conclusion to be true: nuclear waste can contribute to making catalytic converters through a new technique, and catalytic converters remove noxious gases from car exhaust. And Marta doesn't accuse James of making a circular argument, she just points out additional information he hasn't considered.

This is correct. Marta doesn't deny anything James has said, she just adds information he doesn't take into account in his argument: that the new technique has not yet been phased into the manufacture of catalytic converters. Just because this technique has been discovered and has potential benefits does not mean it is currently providing those benefits.

Marta says nothing to suggest she doubts the plausibility of James’s premises. In fact, she concedes that the technique James mentions is effective. She undermines his conclusion by pointing out additional information, not by undermining his premises.