Regarding the objective, it's targeting the most common understanding of "frog" and "toad", which for most people will be folk taxonomy, and they probably aren't aware that there's not a scientific taxonomic distinction. So from your list, I think most people will approach it as option "a". However you might be happy to know that when selecting species, there's a couple that are in there because they belong to taxa that contain both "frogs" and "toads", so "b" would be quite difficult!

In that it goes for "a", it's really intended to show that folk taxonomy, like most ways people categorise things, works until it doesn't. Really important to me to note that it's not trying to favour "scientific" taxonomy over folk, though. There's evidence that practicing folk taxonomy, by observing and categorising what you see in nature around you, helps build connectedness with nature, which has a lot of benefits, and making people aware of folk taxonomy as a thing that they contribute to is the ultimate objective.