ANS:A -

As per Froude Number (Fr):

If Fr < 1 then flow is Sub-Critical flow.
If Fr > 1 then flow is Super-Critical flow,
If Fr = 1 then flow is Critical flow,

So, The formula of Fr is:
Fr = v / ( g d) ^ 1/2.
For critical depth put Fr = 1 and solve for d.
1 = v / ( g d) ^ 1/2,
(g d) ^1/2 = v.

Squaring on Both Sides;
(g d) = v ^2.
d (critical) = (v ^2)/g.