A fast food chain admitted that they donated money to a group that was anti- gay marriage. Many people refused to eat at their establishments and some even picketed the restaurants on weekends. What type of tactic was being used to promote social justice?

Boycott. However, this question is propaganda, not a real question.

Explanation:

American society is polarized. But there is more than one pole. While it is true that "many" people stopped patronizing the fast food chain that donated money to groups opposed to homosexuality, it is also true that many more people shifted their patronage in favor of that same fast food chain for that same reason.

There was no universal boycott of that fast food chain. If anything, it saw its number of dedicated customers increase, rather than the opposite. Not everybody is a left-winger. It's likely that those who are conservatives outnumber the liberals, and always have. And, if recent trends continue, the left will wither and die, as the United States joins most of Europe in moving politically toward the nationalist, populist right.