The correct answer is D. All the above.
A column is a structural member that is subjected to compression and whose dimensions are small as compared to its length. The vertical compression members are generally known as columns or stanchions. Deflection in lateral direction of a long column, is generally known as buckling.
Buckling is a failure mode in which a column or other structural member under compression suddenly collapses due to a lateral deflection. Buckling is caused by the combination of the compressive load and the slenderness of the member. The slenderness of a member is a measure of its length relative to its cross-sectional dimensions.
The buckling load of a column is the maximum load that the column can support without buckling. The buckling load of a column is determined by its length, cross-sectional dimensions, and material properties.
The buckling of columns can be prevented by increasing the column’s cross-sectional area, decreasing its length, or using a material with a higher modulus of elasticity.