Final answer:
Sieving is the correct separation method for separating sand and pebbles because it uses a sieve with holes that allow sand to pass through while retaining pebbles. Option b.
Step-by-step explanation:
The separation method used to separate sand and pebbles is sieving. This method takes advantage of the considerable size difference between the larger pebbles and the finer sand particles. A sieve has holes that allow smaller particles like sand to pass through while retaining larger particles like pebbles. Sieving does not require a liquid medium and is purely based on the physical sizes of the components of the mixture. Option b.
Other common separation techniques include filtration, which is mostly used to separate solids from fluids like in water treatment processes but is not suitable for separating dry mixtures such as sand and pebbles. Similarly, methods such as using a magnet or decantation do not apply here since they target other kinds of separation challenges, like magnetic properties or suspension of particles in a liquid respectively.