The correct answer is: B. 0.35 to 0.60 mm
The effective size of sand particles used in slow sand filters is 0.35 to 0.60 mm. This is because these particles are large enough to trap suspended particles in the water, but small enough to allow water to flow through the filter bed at a reasonable rate.
Option A is incorrect because the effective size of sand particles used in slow sand filters is not 0.25 to 0.35 mm. Option C is incorrect because the effective size of sand particles used in slow sand filters is not 0.60 to 1.00 mm. Option D is incorrect because the effective size of sand particles used in slow sand filters is not 1.00 to 1.80 mm.
The effective size of sand particles used in slow sand filters is important because it affects the efficiency of the filter. If the particles are too small, they will not be able to trap suspended particles in the water. If the particles are too large, they will not allow water to flow through the filter bed at a reasonable rate.