Which material processing technique improves a material's strength without changing its volume?

The correct choice is cold working, as this technique enhances a material's strength without altering its volume significantly. Cold working involves deforming a material at room temperature, which increases its dislocation density and creates a more complex microstructure. This process results in strain hardening, leading to improvements in yield strength and tensile strength while the overall volume of the material remains unchanged.

While heat treatment is a process that can alter the microstructure and properties of a material, it often involves changes in volume due to thermal expansion and contraction during different phases like austenitization and tempering. Similarly, annealing is a heat treatment aimed at relieving stresses and softening materials, resulting in potential changes in volume as well. Quenching, which involves rapidly cooling a hot material to set its microstructure, can also lead to dimensional changes and, therefore, is not focused on maintaining the original volume.

Cold working, however, specifically retains the volume while enhancing strength, making it the ideal choice for this question.