At the origin, the electric field is directed along the positive x...

The relation between electric field $\vec{E}$ and electric potential $V$ is given by $dV = -\vec{E} \cdot d\vec{r}$.

This implies that the electric potential decreases in the direction of the electric field.

Since the electric field at the origin is directed along the positive $x$-axis, the potential decreases as the $x$-coordinate increases.

The points on the circle are $A(a, 0)$, $B(0, a)$, $C(-a, 0)$, and $D(0, -a)$.

Among these points, point $A$ has the maximum $x$-coordinate ($x = a$).

Therefore, the electric potential is minimum at point $A$.