Why is A correct?

Line 24 does indeed shows that neutrinos are a plurality, but this doesn't show that they are a majority. Just because something is the most numerous type of thing, doesn't mean that more than half of all things are of that type. It just means that there is no other type of things that there are more of. For example, I think (and I'm kinda making these stats up so don't go quoting them ) Catholics are the most numerous religion in the US with about 15 percent of the population (although depending on how you slice evangelicals they might be ahead). But this is far from a majority, it's only a plurality. This could be the case with neutrinos as well, so we can't prove they are a majority.