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An analytic function of a complex variable $z = x + i y$ is expressed as $f (z) = u(x, y) + i v(x, y)$ , where $i=\sqrt{-1}$ . If $u(x, y) = x^2 − y^2$ , then expression for $v(x, y)$ in terms of $x$, $y$ and a general constant $c$ would be

- $xy + c \\$
- $\dfrac{x^2+y^2}{2}+c \\$
- $2xy+c \\$
- $\dfrac{(x-y)^2}{2}+c$