The correct option is A 2.5 cm
The height of water in the tank becomes maximum when the volume of water flowing into the tank per second becomes equal to the volume flowing out per second.
Volume of water flowing out per second
=Av=A√2gh....(i)
Volume of water flowing in per second
=70cm3sec....(ii)
From (i) and (ii) we get
A√2gh=70⇒1×√2gh=70
⇒1×√2×980×h=70
∴ h=49001960=2.5cm.