Final answer:
The concept illustrated by an interest group's efforts to ban guns following a mass shooting and succeeding in law changes is Policy advocacy.
Step-by-step explanation:
In the scenario described, the concept illustrated is Policy advocacy. After a mass shooting, an interest group dedicated to banning guns organized several demonstrations and protests that capture national attention. This scenario clearly depicts how an interest group put forth efforts to sway public opinion and lawmaking processes to reflect their values, ultimately leading to changes in law. Policy advocacy is when an organization or group actively campaigns for policy changes. In this case, the efforts of demonstrations, alongside amplifying their cause through social media, allowed the interest group to influence policy changes.