Sufficient assumption questions are by no means my strength so I think it would do me - and hopefully some others - some good if I go through my thought process with this question. That being said, please take my explanation with a grain of salt!
Pretty simple argument we're given here in the stimulus. The core is as follows:
free speech is an activity that is conducive to a healthy nation --> free speech is in the best interest of the people ---> government ought to protect and encourage free speech
We're asked to find an assumption that completely fills in the gap left by the stimulus. If I'm not mistaken, for questions with an intermediate conclusion we can have the gap between either the intermediate conclusion and the final conclusion OR the first premise and the intermediate conclusion.
Answer choices:
A) is our credited response: it provides the link between the intermediate conclusion and the main conclusion. "An activity" encompasses free speech and the direction of the logic is the same as in the stimulus and so we have our sufficient assumption
B) is incorrect because it is a more general version of the conclusion and so provides no missing link
C) helps a government to govern? out of scope
D) is backwards
E) is tempting and probably the trap answer for this Q. But who's to say universal human rights includes free speech?