**Core Concept**
Elastic strain is a measure of deformation of a material within its elastic limit. It is reversible and does not cause permanent damage to the material. The proportional limit is the maximum stress a material can withstand without undergoing plastic deformation.

**Why the Correct Answer is Right**
The correct answer refers to the strain that occurs when a material is stressed to its proportional limit. This is a measure of the material's elastic response to stress. When a material is stressed within its elastic limit, it will return to its original shape once the stress is removed. This is due to the material's ability to resist deformation and maintain its original structure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the strain that occurs when a material is stressed to its proportional limit. The proportional limit is a measure of the maximum stress a material can withstand without undergoing plastic deformation, not a type of strain.

**Option B:** This option is incorrect because it refers to the strain that occurs when a material is stressed beyond its proportional limit. This is a measure of the material's plastic response to stress, resulting in permanent deformation.

**Option C:** This option is incorrect because it is a distractor and does not accurately describe the strain that occurs when a material is stressed to its proportional limit.

**Clinical Pearl / High-Yield Fact**
The proportional limit is an important concept in materials science and engineering, as it helps to determine the maximum stress a material can withstand without failing.

**Correct Answer: C. Elastic strain.**