Observation

This was a popular question among candidates. Many candidates attempted this question but few were able to explain correctly proportionality limit and yield point.
                                   
The expected response is as stated below:

(a) Explanation of proportionality limit
Maximum force/load which a material will withstand for force/load applied to it to be proportional to extension.

OR

Maximum stress beyond which stress and strain are not proportional for an elastic material under tension.                                                                                     

(b) Explanation of yield point
The minimum force for which an elastic material does not return to its original length when the force is removed/ceases to act.
OR

The point beyond the elastic limit in a force/extension graph beyond/after which the material becomes plastic.