The elastic limit is indicated by the point at which elongation:

The elastic limit is a critical concept in materials science and engineering, defined as the maximum stress that a material can withstand without undergoing permanent deformation. When a material is loaded, it will initially deform elastically, meaning that if the load is removed, it will return to its original shape.

Option B specifies that the elastic limit is marked by a sudden great elongation of the specimen. This is a crucial aspect because, as materials are stressed, they exhibit linear elastic behavior up to a certain point. Beyond this point, an increase in stress can lead to a nonlinear response, which may lead to plastic deformation. The term "sudden great elongation" captures the onset of this nonlinear behavior, where the driver of elongation shifts, indicating that the material can no longer return to its original form once the load has exceeded the elastic limit.

Understanding the elastic limit is essential for ensuring materials are used within their safe operating ranges in various engineering and construction applications. If materials exceed this limit, they risk permanent deformation or failure, which could result in significant structural issues.

The other options do not accurately reflect the characteristics of the elastic limit and its effects on a material's behavior under load, as they relate to either post-elastic limit behavior or miss the point