Column buckling: direction of failure based on principal second moments of area A vertical column has two principal moments of inertia Ixx and Iyy about orthogonal centroidal axes. Identify the direction in which the column will tend to buckle first under axial compression. Choose the correct option.

Concept
Euler buckling shows that the critical load for a prismatic column is P_cr = π² E I / (K L)². For a given column (same E, L, end condition K), the smaller principal second moment of area I gives the smaller critical load and hence buckling occurs first about that weaker axis.

Why not the others?
(a) & (b) mention directions relative to the load line, not stiffness; (c) maximum inertia would raise P_cr, resisting buckling; (d) correctly identifies the weak-axis buckling.

Final Answer
about the axis with minimum moment of inertia