Quiz question uncertainty

Question

Hi, I just completed a quiz with the question below - the correct answer according to the quiz is “en.”
La liberté d'expression est un droit fondamental mais il faut ________respecter les limites.
To me, this is saying that we should be respecting the limits of “freedom of expression,” not the limits “of expression.” So shouldn’t the correct pronoun be “la” to reference “la liberté d'expression”? Thanks for helping me to understand this.

Maarten Kamp · Accepted Answer

Brian, 
the relevant expression  is ' .... respecter les limites de qqc '.  
A prepositional phrase commencing with ' de ' that refers to something previously mentioned, can be replaced with the adverbial pronoun ' en ' but not with a direct object pronoun. 
The prepositional phrase  ' ( respecter les limites ) de la liberté d’expression ‘ can be replaced in full by ' en ' which gives  ' .... il faut en respecter les limites '. 
https://progress.lawlessfrench.com/revision/grammar/the-adverbial-pronoun-en-can-replace-a-phrase-introduced-by-de