  We present observations of the optical afterglow of GRB\,170817A, made by the
{\it Hubble Space Telescope}, between February and August 2018, up to one year
after the neutron star merger, GW170817. The afterglow shows a rapid decline
beyond $170$~days, and confirms the jet origin for the observed outflow, in
contrast to more slowly declining expectations for `failed-jet' scenarios. We
show here that the broadband (radio, optical, X-ray) afterglow is consistent
with a structured outflow where an ultra-relativistic jet, with Lorentz factor
$\Gamma\gtrsim100$, forms a narrow core ($\sim5^\circ$) and is surrounded by a
wider angular component that extends to $\sim15^\circ$, which is itself
relativistic ($\Gamma\gtrsim5$). For a two-component model of this structure,
the late-time optical decline, where $F \propto t^{-\alpha}$, is
$\alpha=2.20\pm0.18$, and for a Gaussian structure the decline is
$\alpha=2.45\pm0.23$. We find the Gaussian model to be consistent with both the
early $\sim10$ days and late $\gtrsim290$ days data. The agreement of the
optical light curve with the evolution of the broadband spectral energy
distribution and its continued decline indicates that the optical flux is
arising primarily from the afterglow and not any underlying host system. This
provides the deepest limits on any host stellar cluster, with a luminosity
$\lesssim 4000 L_\odot~(M_{\rm F606W}\gtrsim-4.3)$.
