The correct answer is: C. Reinforcing bars in a column should not be less than 12 mm in diameter.
The minimum cross sectional area of longitudinal reinforcement in a column is 0.8%. This means that for every 100 square centimeters of concrete in the column, there must be at least 0.8 square centimeters of reinforcing steel. The spacing of longitudinal bars measured along the periphery of column should not exceed 300 mm. This means that the bars should be placed no more than 300 mm apart, measured from the center of one bar to the center of the next. The number of longitudinal bars provided in a circular column should not be less than four. This means that there must be at least four bars in a circular column, regardless of the diameter of the column.
The statement “Reinforcing bars in a column should not be less than 12 mm in diameter” is incorrect because there is no minimum diameter requirement for reinforcing bars in columns. The diameter of the bars is determined by the design of the column and the loads that it will be subjected to.