Final answer:
Tensile strength is the key property ensuring stability and strength for steel columns supporting a beam, as it defines the material's ability to withstand pulling forces without breaking.
Step-by-step explanation:
The physical property that ensures the stability and strength of two identical steel columns supporting a level beam is tensile strength. Tensile strength refers to the maximum stress that a material can withstand while being stretched or pulled before breaking. It is crucial for the columns because they experience both tensile and compressive stress when supporting loads, and must do so without deforming permanently. Elasticity is also significant as it allows materials to return to their original shape after the load is removed, provided the elastic limit isn't exceeded. However, it is the tensile strength that will ultimately determine the ability of the steel columns to support significant weights without failing.