I mistakenly diagrammed the first sentence of the stimulus (If understanding a word always involves knowing its definition, then understanding a word requires understanding the words that occur in that definition) as two conditionals. This lead me falsely to answer choice A, instead of E.
Instead, I take it I should have approached the question like this:
If understanding a word always involves knowing its definition, then understanding a word requires understanding the words that occur in that definition:
[Understanding --> Knowing the Definition] --> [Understanding --> Understanding Words in Definition]
CP: ~[Understanding --> Understanding Words in Definition] --> ~[Understanding--> Knowing Definition]
We are further told that there are clearly some people, like all babies, that do not know the dictionary definitions of SOME of the words they utter.
Baby --> ~Understand Dictionary Definitions (of some words!)
In Answer Choice E, we are told
If some babies understand all the words they utter, then understanding a word does not always involve knowing the dictionary definition.
So this scenario would mean that Babies (s) <--> Understand Words
combined with
Babies --> ~Understand Dictionary Definitions
And gives us:
Understand Words (s) <--> ~Understand Dictionary Definitions
Meaning: If it were true, you don't have to know the Dictionary Definition all the time!