The diameter of transverse reinforcement of columns should be equal to one-fourth of the diameter of the main steel rods but not less than A. 4 mm B. 5 mm C. 6 mm D. 7 mm

The correct answer is B. 5 mm.

The diameter of transverse reinforcement of columns should be equal to one-fourth of the diameter of the main steel rods but not less than 5 mm. This is to ensure that the transverse reinforcement is able to provide adequate confinement to the concrete and prevent it from buckling.

Option A is incorrect because the diameter of transverse reinforcement should not be less than 5 mm. Option C is incorrect because the diameter of transverse reinforcement should not be more than one-fourth of the diameter of the main steel rods. Option D is incorrect because the diameter of transverse reinforcement should not be less than 5 mm.