The effective size of sand particles used in slow sand filters is

Slow sand filters are a type of water treatment system.
They use layers of sand to remove impurities from water.
The size of the sand particles is important for how well the filter works.
If the sand is too fine, the water will flow too slowly.
If the sand is too coarse, it won't trap enough impurities.
The effective size refers to the size of the particles where 10% of the sand is finer.
For slow sand filters, the effective size needs to be just right for optimal performance.
The correct range for effective sand particle size in slow sand filters is 0.25 to 0.35 mm.
Therefore, the best option is A.