Minimum spacing between horizontal parallel reinforcement of different sizes, should not be less than A. One diameter of thinner bar B. One diameter of thicker bar C. Twice the diameter of thinner bar D. None of these

The correct answer is: A. One diameter of thinner bar.

The minimum spacing between horizontal parallel reinforcement of different sizes should not be less than one diameter of the thinner bar. This is to ensure that the bars are properly spaced and that the concrete can flow freely around them. If the bars are too close together, the concrete may not be able to flow properly, which could lead to problems with the strength of the structure.

Option B is incorrect because it is the diameter of the thicker bar, not the thinner bar. Option C is incorrect because it is twice the diameter of the thinner bar. Option D is incorrect because it is not a valid option.