b
(b) From Hooke's law, Young modulus $Y=\frac{S t r e s s}{S t r a i n}$
The Strain produced in a perfectly rigid body is zero i.e Strain$=0$
$\therefore Y=\frac{S t r e s s}{0} \quad \Longrightarrow Y=\infty$
Thus the Young's modulus of perfectly rigid body material is infinite.