Question

For perfect rigid body young's modulus is

Answer 1

FOR A PERFECT RIGID BODY YOUNGS MODULUS IS taken as infinite as strain is taken as zero . Because there is no deformation in the body when it it is subjected to a force or torque.
 

Answer 2

Young's modulus for a perfect rigid body is infinity.
Young's modulus = stress / strain  (Hooke's law)
As there is no strain produced strain = 0
Young's modulus = stress/0 = infinity.