Answer
$None$ $of$ $the$ $solutions$ $are$ $extraneous$
Work Step by Step
$Given,$ $y=\sqrt (2y)$
Checking for $y=0$ :
$L.H.S=0$
$R.H.S=\sqrt(2X0)=\sqrt 0=0$
$L.H.S=R.H.S$,hence $y=0$ is not extraneous
Checking for $y=2$:
$L.H.S=2$
$R.H.S=\sqrt (2X2)=\sqrt 4=2$
$L.H.S=R.H.S$,hence $y=2$ is not extraneous