CBSE Class 11-science Answered

We know,

Young's modulus = Stress/Strain 

The assertion that a perfectly plastic body cannot regain its original shape when pressure is applied is a correct statement. This is related to the fact that Young’s modulus of a perfectly plastic body is zero.

Furthermore, the reason statement aligns with this assertion. In a perfectly plastic body, the restoring force generated is indeed zero, which explains the inability of these materials to revert to their original shape. Therefore, the reason statement is true and is the correct explanation of the assertion statement.

Thus, we can say, both assertion and reason are true and reason is the correct explanation of assertion.