6) C

The main conclusion is the last line of the passage.

A mainly focuses on the regulation of social media whereas the conclusion deals with the relationship between social media and democracy. So A is wrong.

The last line uses the term ‘social media’ whereas option B uses the term ‘technological developments’. This is why B is wrong.
C is correct because the last line states that social media has “undermined democracy”. Undermined means ‘lessen the effectiveness of’. So social media has basically made democracy weaker. C states this.

D mainly focuses on the availability of truthful information whereas the conclusion deals with the relationship between social media and democracy. So D is wrong.

E mainly focuses on organised protests and direct action whereas the conclusion deals with the relationship between social media and democracy. So E is wrong.