The minimum cover to the ties or spirals should not be less than A. 15 mm B. 20 mm C. 25 mm D. 50mm

The correct answer is A. 15 mm.

The minimum cover to the ties or spirals should not be less than 15 mm. This is to ensure that the concrete is able to provide adequate protection to the reinforcement from corrosion. The cover also helps to distribute the load evenly over the reinforcement, preventing it from buckling.

Option B is incorrect because 20 mm is not the minimum cover required by code. Option C is incorrect because 25 mm is not the minimum cover required by code. Option D is incorrect because 50 mm is not the minimum cover required by code.