The correct answer is D. All the above.
The minimum number of main steel bars provided in R.C.C. columns is 4, regardless of the shape of the column. This is because the main steel bars provide the main tensile strength of the column, and it is important to have a minimum number of bars to ensure that the column is able to carry the loads it is designed for.
In rectangular columns, the main steel bars are typically placed in two rows, with two bars in each row. This arrangement provides the best balance of strength and stiffness.
In circular columns, the main steel bars are typically placed in a helical pattern. This arrangement provides good strength and stiffness, and it also helps to distribute the load evenly around the column.
In octagonal columns, the main steel bars are typically placed in a star pattern. This arrangement provides good strength and stiffness, and it also helps to distribute the load evenly around the column.
It is important to note that the minimum number of main steel bars provided in R.C.C. columns is just a guideline. The actual number of bars required will depend on the specific design of the column.