XD - Fun!

Did you just keep trying/submitting this until it eventually passed?

I had the same misunderstanding.
I took this ("multiplicities") to be an elaboration/another way of saying "the elements in b are the elements in a squared".
But looking at the definition of the word, "In mathematics, the multiplicity of a member of a multiset is the number of times it appears in the multiset" - I can see now that it was explicit.

For what it's worth, I am a native English speaker, but "multiplicities" is not commonly used in everyday language, but perhaps in the field of mathematics it is.