Which of these invertebrate pairs is mismatched?

I think the question means to ask which one out of the list is the odd one out. The answer is the last one obviously - it is not a mistake.

Spicules are spike-like structures found on many organisms and characteristic of sponges. Spicules are 'parts' of an organism.

Nematocysts are again an integral 'part' of Cnidarians. The same goes for proglottids (segments) and cilia (hair-like organelles).

All the above are parts of organisms. None of them exist as individual organisms on their own. You could basically say what a limb is to a human, a spicule is to a sponge, for example.

However, Polyp and Medusa (jellyfish) are entire forms of organisms in the phylum Cnidaria (no doubt though, as you say, both of them ARE Cnidarians but that's not what the question is asking!). They can exist as individual organisms and do not constitute any 'part' of an individual organism. So thats the odd choice, individuals do not belong in a list of organelles.

I think the answer is pretty easy to spot if you know the meaning of each of the terms. I know the answer is not that simple but it is kindof like: hand, arm, leg, feet, monkey. Which one is the odd one out?

Roundworms - Cilia

I'm presuming the writers and editors made a mistake.