What is Proportional Limit?

# The proportional limit is defined as:
A. The maximum stress in a structure
B. The minimum force required to cause a structure to break
C. The maximum stress that can be induced without permanent deformation
D. The maximum elongation under tension that
 can be measured before failure

The correct answer is C. The maximum stress that can be induced without permanent deformation.

Stresses above proportional limit cause plastic deformation. This stress is called yield strength or proof stress. It  is the greatest stress, produced in a material such that stress is directly proportional to the strain. 

Phillip's Science of Dental Materials, 12th ed. Page no 57.