The square root of the ratio of moment of inertia of the cross-section to its cross-sectional area is called

Consider the following statements:
$$1.$$ On a principal plane, only normal stress acts
$$2.$$ On a principal plane, both normal and shear stresses act
$$3.$$ On a principal plane, only shear stress acts
$$4.$$ Isotropic state of stress is independent of frame of reference.

Which of the above statements is/are correct ?

Match List- $${\rm I}$$ (Shear Force Diagrams) beams with List- $${\rm II}$$ (Diagram of beams with supports and loading) and select the correct answer by using the codes given below the lists:

The point within the cross sectional plane of a beam through which the resultant of the external loading on the beam has to pass through to ensure pure bending without twisting of the cross-section of the beam is called