Which language L have exactly one equivalence class

Question

Consider the alphabet {a,b}, for which language does the equivalence relation R have exactly one equivalence class?

From what i understand about equivalence class, each state is consider a class. So would $a^n : n \ge 0$ be the one class ? Can someone help me understand where to begin?

Answer 1

It is probably the Nerode equivalence relation: (Link) $$ x \sim_L y \iff (\forall z \in \Sigma^*: \ xz \in L \iff yz \in L) $$