The correct answer is: C. 10 mm.
The diameter of main bars in R.C.C. columns shall not be less than 10 mm. This is to ensure that the column has sufficient strength to carry the loads imposed on it. The diameter of the main bars is also important for the development of bond between the concrete and the steel. If the diameter of the main bars is too small, the bond may not be adequate and the column may fail.
Option A: 6 mm is too small a diameter for main bars in R.C.C. columns. It is not recommended to use main bars with a diameter of less than 10 mm.
Option B: 8 mm is a common diameter for main bars in R.C.C. columns. However, it is not the minimum diameter that is required.
Option D: 12 mm is a larger diameter than is typically used for main bars in R.C.C. columns. It is not necessary to use main bars with a diameter of 12 mm, and doing so may increase the cost of the column without providing any additional benefits.