They’d also been talking about the injustices they experienced daily, like not being able to eat at a lunch counter

Part of the sentence points to the correct definition of injustice in the sentence?

Yes, the phrase "not being able to eat at a lunch counter" exemplifies an injustice.

The correct definition of injustice in the sentence is "not being able to eat at a lunch counter."

Yes, in this sentence, the word "injustices" refers to the difficulties or unfair treatments that the individuals are going through, particularly the fact that they are not allowed to eat at a lunch counter.