The correct option is B Sieving
Sieving is used to separate a solid-solid mixture, where components are very small in size. To carry out sieving, a sieve is used. The pore size of seive is such that it allows the slightly smaller component to pass through it while the bigger component stays at the top of sieve.
Grains are larger in size than flour, so flour will pass through the sieve, leaving the grains behind. So, grains can be separated from flour by sieving.