Answer: (iii) All strings over $\{a, b\}$, including $\varepsilon$

Explanation

• $(a \mid b)$ matches exactly one symbol: either $a$ or $b$.
• The Kleene star $^*$ allows zero or more repetitions of that choice.

So the language is $\{\varepsilon, a, b, aa, ab, ba, bb, aaa, \dots\}$, i.e. every string over the alphabet $\{a,b\}$, with $\varepsilon$ (zero repetitions) included.

Why the other options fail:

• (i), (ii): they allow only one of the two symbols.
• (iv): it forces the string to start with $ab$, but $(a\mid b)^*$ also accepts $b$, $\varepsilon$, etc.

$$\boxed{\text{(iii)}}$$