Columns: define slenderness ratio precisely. Use the standard mechanical design definition relating effective length and cross-section property. Choose the correct option.

Definition
The slenderness ratio quantifies column propensity to buckle and is defined as λ = L_e / k_min where L_e is the effective length and k_min is the least radius of gyration of the cross section.

Why option (c)
Euler buckling and design formulas employ λ = L_e/k; using the least radius of gyration is conservative because buckling occurs about the weakest axis.

Final Answer
Effective length of column ÷ least radius of gyration of the column