The moment of inertia of a thin rod about an axis passing through its mid point and perpendicular to the rod is $2400\, g\, cm^2$ . The length of the $400\, g $ rod is nearly:

8.5 cm

The correct answer is option (1) : 8.5 cm

Moment of inertia of road $=I=\frac{ml^2}{12}$

$⇒2400=400\frac{l^2}{12}$

$⇒72=l^2$

$⇒l=\sqrt{72}=8.48\, cm ≈8.5\,  cm $