Answer
Graph II.
Work Step by Step
Because $\sin^{2}A+\cos^{2}A=1$, the x and z coordinates satisfy
$x^{2}+z^{2}=t^{2}$
and the points on the curve satisfy
$x^{2}+z^{2}=y^{2}$
so the curve will spiral into ever increasing circles as y increases.
(this describes a spiral along a circular cone opening along the y-axis).
The additional condition, $t\geq 0$
means that the propagation takes place only along the positive y axis.
This is graph II.
