DSE 2018 Question 4: Real analysis

Let $\mathbb{R}$ be the set of real numbers and let $\mathcal{D}$ be the set of functions $d : \mathbb{R} \times \mathbb{R} \to \mathbb{R}$ that satisfy, for all $x, y, z \in \mathbb{R}$: (i) $d(x,y) \geq 0$; (ii) $d(x,y) = 0$ if and only if $x = y$; (iii) $d(x,y) = d(y,x)$; (iv) $d(x,z) \leq d(x,y) + d(y,z)$. Which of the following is not a function in $\mathcal{D}$?