For a distant galaxy moving at a non-relativistic speed, its redshift $z$ is related to its recessional velocity $v$ by $z=\frac{v}{c}$, where $c$ is the speed of light. According to Hubble's law, $v=H_{0}d$, where $d$ is the distance to the galaxy and $H_0$ is the Hubble constant.

If a cosmologist plots a graph with redshift $z$ on the vertical axis against distance $d$ on the horizontal axis (using consistent SI units), which of the following expressions represents the gradient of the line of best fit?