With which of the following electronic configuration an atom has the lowest ionisation enthalpy?

The factors on which the ionisation enthalpy depends are:
(i) Atomic size: the ionisation enthalpy decreases as the atomic size increases, because the attractive force of the nucleus on the outer electron decreases.
(ii) Number of electrons in the inner shells (screening effect): as the number of shells increases, the screening effect also increases, and hence the ionisation enthalpy tends to decrease.
In $1s^2\,2s^2\,2p^5\,3s^1$ the outermost electron is in the third shell (3s). It is farther from the nucleus and is well shielded by the inner electrons, so it is removed most easily.
(This is an excited state of neon, in which one 2p electron has been promoted to 3s.)
In the other three configurations the electron has to be removed from the second shell, which is close to the nucleus; $2p^6$ is a stable noble gas configuration and $2p^3$ is a stable half-filled configuration, so these need much more energy.
So the lowest ionisation enthalpy belongs to $1s^2\,2s^2\,2p^5\,3s^1$.