Integer M and N has 20 factors in which r and s are their respective factors
Factors of 20 are;
$$20\cdot1=20$$ k/
$$10\cdot2=20$$
$$4\cdot5=20$$
$$2\cdot2\cdot5=20$$
In all the possible cases listed above, there is at least one prime number in a case and at most three prime numbers in a case.
maximum vale of c-s = 3-1=2

$$answer\ is\ Option\ C$$