b) Decidable
R and S are regular expressions, it means they will generate regular languages only.
Whether a regular language L1 is a subset of another regular language L2 is a decidable Problem.
L1 is subset of L2, it means L2 can generate all strings generated by L1.
L= L1-L2 , then L should be an empty language if L1 is subset of L2.
L= L1 Intersection L2c
Complement of a regular language is regular itself, hence Complement of L2 will be a regular language, regular languages are closed under Intersection hence L will be regular too.
it's a Decidable Problem whether a DFA accepts an empty language or non-empty.