Which of these represents a regular language?

(i) $\{ a^n b^n \mid n \ge 0 \}$
(ii) Strings with equal number of $a$’s and $b$’s
(iii) $\{ a^n \mid n \text{ is prime} \}$
(iv) $\{ w \mid w \text{ starts with 'a' and ends with 'b'} \}$