The correct answer is: B. Yield stress
The yield stress is the stress at which a material begins to deform permanently. It is a measure of the material’s strength and is used to identify the quality of structural steel.
The ultimate stress is the stress at which a material fails. It is a measure of the material’s toughness and is not used to identify the quality of structural steel.
The proof stress is a stress that is lower than the yield stress. It is used to determine the yield strength of a material in the presence of residual stresses.
None of the above is the correct answer.