Answer: B . particle size
Explanation:
Let's begin by explaining that a molecular sieve is a device, whose composition allows the absorbtion of molecules that are small enough to pass through its pores, since it is made up of small pores of a precise and uniform size.
Thus, very small molecules manage to pass through the pores of the sieve, while large ones do not.
So, based on this premise, a molecular sieve separates the substances by their size.
Hence, the correct option is B.