According to Hubble's law, the recessional velocity $v$ of a distant galaxy is proportional to its distance $d$ from Earth, such that $v=H_{0}d$, where $H_0$ is the Hubble constant. For relatively nearby galaxies, the cosmological redshift $z$ is given by $z\approx \frac{v}{c}$, where $c$ is the speed of light in a vacuum.

If a graph of redshift $z$ (on the vertical axis) is plotted against distance $d$ (on the horizontal axis), which of the following expressions represents the gradient of the line of best fit?