Answer
Circle
Work Step by Step
If $B=0$ and $A $ and $C$ are not both zero, then the general equation of a conic has the form of $Ax^2+Cy^2+Dx+Ey+F=0~~(1)$
(a) When $AC=0$, then a conic defines a parabola.
(b) When $AC \gt 0$, then a conic defines an ellipse and $A\ne C$
(c) When $AC \gt 0$, then a conic defines a Circle and $A=C$
(d) When $AC \lt 0$, then a conic defines a hyperbola.
We have: $A=1,C=1$
Plug these values in Equation (1) to obtain:
$AC=(1)(1)=1\gt 0$, and $1=1$, so the conic represents a Circle.