Correct Answer - Option 3 : Triaxial compression
The concrete inside the core is subjected to tri-axial compression. It is confined by hoop stresses in spiral reinforcement and axial load along the longitudinal direction.
Uniaxial compression acts only if plain short column loaded axially.
Bending and compression acts when reinforced short column is loaded unilateral eccentricity or bilateral eccentricity.
acts when short column is reinforced with vertical stirrups and loaded axially.