The questions asks for possible translations of "Liliane's son, whom I told you about, lives in Angers."?The following option is marked incorrect, but I don't understand why.
Le fils de Liliane, qui je t'ai parlé de, habite à Angers.
I get that I need to distinguish between Liliane and her son, so the best option is to use "duquel", but if "dont" and "à qui" are accepted, why is "qui" not accepted?
The lesson covers this - your suggested wording leaves a 'dangling preposition' (de), at the end of the clause. "Qui" on its own is incorrect as the verb form is 'parler de'.
Trailing or dangling preposition use is common in English, but apart from a few exceptions in informal, everyday speech, is not done in French. Worth another look at the "TIPS" section. The reason it is this way - "because it is", is the usual answer to any questions regarding fundamental rules of grammar in any language.
'Dont' would be the most commonly used word/expression here in speech - the person being spoken to will know if they were previously spoken to about Liliane or Liliane's son, and in the unlikely event of being uncertain would simply seek clarification. Neither 'de qui' nor the even less commonly used 'duquel' would be wrong though.
To complement Maarten's answer with a few examples:
Hugo, dont je t'ai parlé, habite à Angers. (That's the preferred version.)
Hugo, de qui je t'ai parlé, habite à Angers. (Also possible but less elegant.)
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