The force constant (K) of a spring is defined by Hooke's Law, F = -Kx, where F is the restoring force and x is the displacement. For a material, it is related to Young's modulus (Y), cross-sectional area (A), and original length (L). The formula is derived from the stress-strain relationship.

The correct expression is: $\frac{YA}{L}$, where Y is Young's modulus, A is area, and L is length.

Final Answer: $\frac{YA}{L}$