This isn’t an entirely unreasonable inference. It’s wrong because it’s much less well supported than (B), not because it’s completely unsupported.

The main assumption (A) makes is that we previously thought Halley’s was a somewhat normal comet and not some crazy outlier. Like if we used to think Halley’s is made of iron, probably there are some other comets also made of iron.

But the stimulus never discusses other comets at all – we’ve only got information comparing our old understanding of Haley’s to our new one. So the inference about other comets is based purely on facts we’re bringing in from the outside. It might be a fair guess in the real world, though.

Confirming (B) means tracking all the UP and DOWN relationships.

See the analysis section for a walkthrough of why learning that Halley’s comet is less reflective than we thought means it’s bigger than we thought (according to our estimate).

The amount of light reflected off the comet has presumably stayed the same. At least, we have no indication that we’re seeing more or less light than before. What has changed is the inference we make based on the light we’re seeing.

Similar to (A), there’s some reason to believe that our previous baseline understanding of how reflective comets are was based on knowledge of other comets that are more reflective than Halley’s. That’s not an entirely unreasonable inference – it’s just much less well supported than (B).

“Good” should ring some alarm bells, since it sets a normative standard – a value judgement about how estimates ought to be. It’s not a moral or ethical judgment, of course – it’s more a judgment about what counts as proper science – but that’s okay. What’s important is that the stimulus features only descriptive claims about what the estimate is, without commenting on what makes estimates good or bad.