Correct Answer - Option 3 : 12 mm
As per IS 456: 2000, clause 188.8.131.52,
Longitudinal reinforcement in a column:
The minimum diameter of longitudinal reinforcement bars:
- The main longitudinal reinforcement bars used in the column shall not be less than 12 mm in diameter.
The minimum number of longitudinal bars:
The minimum number of the longitudinal bar provided in the column shall be
- four in rectangular columns
- six in circular columns
Percentage of steel:
- The cross-sectional area of longitudinal reinforcement shall be not less than 0.8 percent nor more than 6 percent of the gross cross-sectional area of the column.
- A reinforced concrete column having helical reinforcement shall have at least six-bar of longitudinal reinforcement within the helical reinforcement.
- The spacing of longitudinal bars measured along the periphery of the column shall not exceed 300 mm.